Compositions and methods for detection and measurement of rna modifications through targeted rna editing

ABSTRACT

Provided herein are compositions and methods for detection of N 6 -methyladenosine (m 6 A) in ribonucleic acid (RNA). The provided compositions include fusion proteins that can be used to edit RNA and detect m6A residues. Also provided are nucleic acids, vectors, constructs, host cells, and transgenic animals that encode or express such fusions proteins.

PRIOR RELATED APPLICATION

This application claims the benefit of and priority to U.S. Provisional Application No. 62/853,768 filed on May 29,2019, which is hereby incorporated by reference in its entirety

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made with government support under Grant Nos. 1DP1DA046584-01 and 1R01MH118366-01 awarded by the National Institutes of Health/National Institute on Drug Abuse and National Institutes of Health/National Institute of Mental Health, respectively. The government has certain rights in the invention.

FIELD

This disclosure describes compositions and methods for detecting RNA modifications in cells and tissues.

REFERENCE TO A SEQUENCE LISTING SUBMITTED AS A TEXT FILE VIA EFS-WEB

The official copy of the sequence listing is submitted electronically via EFS-Web as an ASCII formatted sequence listing with a file named 1185352_seqlist.txt, created on May 27, 2020, and having a size of 49.4 KB and is filed concurrently with the specification. The sequence listing contained in this ASCII formatted document is part of the specification and is herein incorporated by reference in its entirety.

BACKGROUND

N⁶-methyladenosine (m⁶A) is the most abundant internal ribonucleic acid (RNA) modification and has been implicated in nearly every aspect of the RNA life cycle. Moreover, proteins involved in the formation, removal, and recognition of m⁶A have emerged as critical regulators of a variety of physiological processes, ranging from stem cell proliferation to learning and memory. To date, methods for the detection of m⁶A have relied on immunoprecipitation of methylated RNAs using m⁶A -recognizing antibodies. However, these methods suffer from several limitations. For example, m⁶A antibodies also recognize the structurally similar cap modification, m⁶Am, so immunoprecipitation of methylated RNAs does not exclusively enrich for m⁶A containing RNA. Also, a large amount of input RNA is required for immunoprecipitation and library preparation, which makes global m⁶A detection prohibitive for limited quantity samples. Finally, antibody-based approaches are costly and the associated library preparation steps are time-consuming, which can be a limiting factor for many experiments.

SUMMARY

Provided herein are fusion proteins that can be used to edit RNA and detect m⁶A residues. The fusion proteins comprise an N⁶-methyladenosine (m⁶A) binding domain of a YT521-B homology (YTH) domain-containing protein fused to a catalytic domain of a cytidine deaminase or a catalytic domain of an adenosine deaminase. In some embodiments, the m⁶A binding domain is fused to the catalytic domain via a peptide linker.

In some embodiments, the m⁶A binding domain comprises a polypeptide having at least 95% identity to SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 10, or SEQ ID NO: 11.

In some embodiments, the catalytic domain comprises a polypeptide having at least 95% identity to SEQ ID NO 12 or a catalytic fragment thereof, SEQ ID NO: 13 or a catalytic fragment thereof; SEQ ID NO: 14 or a catalytic fragment thereof; or SEQ ID NO: 15.

In some embodiments, the fusion protein further comprises a localization element. In some embodiments, the localization element is fused to the N-terminus or the C-terminus of the fusion protein.

Also provided is a recombinant nucleic acid encoding any of the fusion proteins described herein. Further provided is a DNA construct comprising a promoter operably linked to the recombinant nucleic acid. In some embodiments, the promoter is an inducible promoter. Also provided is a vector comprising a recombinant nucleic acid or DNA construct described. A host cell comprising a recombinant nucleic acid, DNA construct or vector described herein is also provided. In some embodiments, the host cell is a mammalian cell. Also provided is a non-human transgenic animal comprising a host cell described herein.

Also provided is a method for detecting m⁶A residues in the RNA produced by one or more cells comprising: (a) isolating RNA from one or more cells that expresses a fusion protein described herein; (b) amplifying one or more target sequences in the isolated RNA; and (c) sequencing the one or more target sequences to identify cytidine to uridine deamination at sites adjacent to m⁶A residues, thus detecting the m⁶A residues in the RNA of the one of more cells. In some embodiments, the recombinant nucleic acid encoding the fusion protein is introduced into the one or more cells prior to isolating RNA from the cell. In some embodiments, the fusion protein is stably or transiently expressed in the cell.

Also provided is a method for detecting m⁶A residues in the RNA produced by one or more cells comprising: (a) fixing a cell or tissue that expresses a fusion protein described herein; and (b) detecting cytidine to uridine deamination in the RNA, wherein cytidine to uridine deamination is detected via mutation-sensitive in situ hybridization.

Further provided is a method for detecting m⁶A residues in a biological sample comprising:(a) isolating RNA from a biological sample; (b) contacting the RNA with a fusion protein described herein; (c) amplifying one or more target sequences in the RNA; and (d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues in the one or more target sequences, thus detecting the m⁶A residues in the one or more target sequences.

Also provided is method for diagnosing a disease in a subject comprising: (a) isolating RNA from a biological sample; (b) contacting the RNA with a fusion protein described herein; (c) amplifying one or more target sequences in the RNA; and (d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues in the one or more target sequences to identify a pattern of m⁶A residues, a difference in the pattern of m⁶A residues as compared to a reference pattern of m⁶A residues from a reference population(s), indicating the subject has or is at risk for developing the disease.

Further provided is a method for determining the efficacy of a selected treatment for a disease in a subject comprising: (a) isolating RNA from a biological sample from the subject before the selected treatment; (b) contacting the RNA with the fusion protein as described above, (c) amplifying one or more target sequences in the RNA; (d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues to identify a pattern of m⁶A residues in the one or more target sequences; (e) treating the subject with the selected treatment, (f) isolating RNA from a biological sample from the subject at one or more times after administration of the selected treatment; (g) amplifying one or more target sequences in the RNA of step f); (h) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues to identify a pattern of m⁶A residues in the one or more target sequences of step g); and (i) comparing the pattern of m⁶A residues identified in step (d) and (h) to determine whether the pattern of m⁶A residues is the same or whether the m⁶A residues detected in step (d) or (h) is more similar to a reference pattern, a pattern of m⁶A residues in step (h) more similar to the reference pattern indicating the selected treatment is effective for treating the disease in the subject.

In some embodiments, the disease is selected from the group consisting of cancer, an autoimmune disorder, a neurodegenerative disorder and a viral infection. In some embodiments, the biological sample is a biopsy. In some embodiments, the one or more target sequences are amplified by reverse transcriptase polymerase chain reaction (RT-PCR). In some embodiments, cytidine to uridine deamination is identified by sequencing the one or more target sequences. In some embodiments, the RNA comprises one or more RNAs selected from the group consisting of messenger RNA (mRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), a regulatory RNA, a transfer-messenger RNA (tmRNA), ribosomal RNA (rRNA), microRNA (miRNA), long noncoding RNA (lncRNA) or circular RNA (circRNA).

Also provided is a kit comprising: (a) one or more fusion proteins described herein; and (b) one or more primers for amplification of one or more target RNA sequences.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic of the DART-Seq (deamination adjacent to RNA modification targets) approach for m⁶A detection. APOBEC1 is fused to the YTH domain to guide Cytidine (C) to uridine (U) editing at cytidine residues adjacent to m⁶A sites. APOBEC1-YTH is expressed in cells and total RNA is isolated and subjected to RNA-Seq. C to U mutations are then detected to identify sites of m⁶A. An unedited sequence is shown (TACTAGGACGCACCTTA) (SEQ ID NO: 56). Edited sequences are shown as (TACTAGGATGCACCTTA) (SEQ ID NO: 57).

FIG. 2 shows western blot results for anti-HA indicating the levels of APOBEC1-YTH or APOBEC1 -YTH^(mut) in in vitro DART-Seq assays. APOBEC1-YTH or APOBEC1 -YTH^(mut) proteins were synthesized in vitro and an aliquot equivalent to the amount used for DART-Seq assays was removed and subjected to western blot analysis. General protein levels are also shown using ponceau S staining of the membrane. Images are representative of n=2 independent experiments.

FIG. 3 shows APOBEC1-YTH^(mut) exhibiting substantially reduced binding to m⁶A in RNA pulldown assays. Anti-HA western blot results are shown after mixing HA-tagged APOBEC1-YTH or APOBEC1-YTH^(mut) with A- or m⁶A-containing biotinylated RNA and subsequently purifying RNA-bound protein with streptavidin pulldown. 2% of input is shown as a reference. Blots are representative of n=2 independent experiments.

FIG. 4 shows a Venn diagram illustrating overlapping C to U editing sites in DART-Seq libraries were prepared from cells expressing APOBEC1-YTH^(mut) or APOBEC1 alone.

FIG. 5 shows the distribution of exon lengths for internal exons containing C to U editing sites in APOBEC1-YTH-expressing cells. Exon lengths of internal exons from the human genome (hg19 annotation) are shown for comparison. There is a greater proportion of long internal exons among C to U edited exons in APOBEC1-YTH-expressing cells compared to the natural distribution throughout the transcriptome.

FIG. 6A shows results of immunoprecipitation-based m⁶A mapping from three different datasets (PA-m⁶A-Seq15, m⁶A-Seq4, and MeRIP-Seq3) where DART-Seq sites were compared to m⁶A peak regions. In addition, CIMS/CITS sites from two single-nucleotide resolution miCLIP datasets5,6 were also compared to m⁶A peak regions from the three datasets. Shown is the overlap between DART-Seq (left column for each dataset) and the two miCLIP datasets (middle and right columns for each dataset) reported as the percentage of m⁶A peak sites that overlap. Across all three datasets (PA-m⁶A-Seq, m⁶A-Seq, and MeRIP-Seq), DART-Seq performs similarly to miCLIP in identifying m⁶A sites.

FIG. 6B shows DART-Seq C to U editing events (left column) which overlapped with CIMS/CITS sites at non-DRACH m⁶A residues (m⁶A sites not located within a DRACH consensus motif taken from the Ke et al. 2015 dataset) shown as a percentage of the total number of non-DRACH m⁶A sites. CIMS/CITS sites from Linder et al. 2015 were also compared to these non-DRACH sites, showing a similar degree of overlap as is observed with DART-Seq.

FIG. 6C shows minimal promiscuous C to U editing at non-m⁶A-adjacent cytidines as revealed by analysis of DART-Seq site clustering . Individual DART-Seq or CIMS/CITS sites were analyzed from DART-Seq data or two miCLIP datasets (Ke et al. 2015 and Linder et al. 2015). For each site within the dataset, the closest C to U editing site (DART-Seq) or CIMS/CITS site (miCLIP) was identified. Shown is the proportion of total sites (DART-Seq in left hand column; miCLIP in middle and right columns) that are at least 10 nt or 20 nt away from the closest site. All three datasets show a similar distribution of sites, with the majority of sites being at least 10 nt apart.

FIG. 7A shows a pie chart illustrating the distribution of C to U editing events identified by DART-Seq in METTL3 KD cells. n=2 independent samples.

FIG. 7B shows a venn diagram illustrating the overlap between C to U editing sites called by DART-Seq in wild type (WT) and METTL3 KD HEK293T cells.

FIG. 8 shows that low-input DART-Seq performs comparably to high-input DART-Seq in its ability to identify m⁶A sites. DART-Seq datasets from samples prepared using 10 ng (n=1), 100 ng (n=1), and 1 μg (n=3) of total RNA as input were compared to m⁶A peak regions from three independent m⁶A immunoprecipitation datasets (m⁶A-Seq4, PA-m⁶A-Seq15, and MeRIP-Seq3). The proportion of m⁶A peaks from each dataset that overlaps with each of the three DART-Seq datasets is shown.

FIG. 9A shows APOBEC1-YTH expression does not affect cell viability. Trypan blue staining of HEK293T cells transfected with APOBEC1-YTH was used to assess cell viability. The proportion of viable cells is shown compared to untransfected HEK293T cells (“Control”). Shown are sample means +/−S.D. n=4 biological replicates; “n.s” =non-significant (P=0.943); 2-tailed t-test.

FIG. 9B shows APOBEC1-YTH expression does not affect levels of the apoptosis marker cleaved caspase 3. HEK293T cells were transfected with APOBEC1-YTH and protein was isolated 24 h later. Western blotting was done to assess levels of cleaved caspase 3. Shown are the average levels of cleaved caspase 3 relative to β-actin. Compared to untransfected HEK293T cells (“Control”), APOBEC1-YTH-expressing cells do not exhibit altered levels of cleaved caspase 3. Shown are sample means +/−S.D. n=4 biological replicates; “n.s” =non-significant (P=0.462); 2-tailed t-test.

FIG. 9C shows levels of major m⁶A regulators are not altered by APOBEC1-YTH expression. Western blotting was used to assess levels of METTL3 and YTHDF2 in APOBEC1-YTH-transfected cells as well as untransfected cells (“Control”). Shown are representative images from n=2 biological replicates.

FIG. 10 shows a density plot for a metagene analysis of the absolute distance of C to U editing sites from the stop codon (position 0) for in vitro DART-Seq assays. C to U editing is enriched in the vicinity of the stop codon, which mirrors the distribution of m⁶A. n=2 independent samples.

FIG. 11 shows quantification of the proportion of U/C editing at the cytidine adjacent to the m⁶A site reveals a direct relationship between the amount of C to U editing and the abundance of m⁶A within the RNA.

FIG. 12A shows the distribution of C to U mutations discovered by DART-Seq in HEK293T cells treated with camptothecin (CPT; n=5,689) compared to untreated controls (UT; n=40,594), indicating a slight enrichment within the CDS (bottom box of each column).

FIG. 12B shows enrichment of m⁶A in the BPTF and ATRX mRNAs following CPT treatment confirmed by MeRIP-RT-qPCR. n=2 biological replicates; box plot indicates mean and upper/lower limits.

FIG. 12C shows a decrease in abundance of the BPTF and A TRXtranscripts following CPT treatment as indicated by RT-qPCR analysis using RNA from untreated and CPT-treated cells. n=2 biological replicates; box plot indicates mean and upper/lower limits.

FIG. 13 shows protein levels of APOBEC1-YTH and APOBEC1-YTHmut. Coomassie staining shows the results of small-scale purification of the APOBEC1-YTH and APOBEC1-YTH^(mut) proteins using bacterial expression and a 6×His/nickel-based purification system.

FIG. 14 shows the percentage (shown as % C to T conversion in cDNA sequences) for each fusion protein variants. The AID-YTH variant causes higher editing rates in the EEF2 mRNA compared to APOBEC1 and APOBEC3A.

FIG. 15 shows the detection of alternative methylation of synaptic mRNAs from mouse brain tissue. Shown are Sanger sequencing traces at two m⁶A sites in the 3′UTR ofArc, an mRNA enriched near synapses. Arrows indicate the cytidine adjacent to the m⁶A site. DART-seq induced C to U mutations (C to T in cDNA) show that site A1682 is selectively methylated in whole brain samples, whereas site A1469 is selectively methylated in the SN fractions, suggesting that DART-seq can detect localization-specific methylation.

DETAILED DESCRIPTION

The following description recites various aspects and embodiments of the present compositions and methods. No particular embodiment is intended to define the scope of the compositions and methods. Rather, the embodiments merely provide non-limiting examples of various compositions and methods that are at least included within the scope of the disclosed compositions and methods. The description is to be read from the perspective of one of ordinary skill in the art; therefore, information well known to the skilled artisan is not necessarily included.

Articles “a” and “an” are used herein to refer to one or to more than one (i.e. at least one) of the grammatical object of the article. By way of example, “an element” means at least one element and can include more than one element.

“About” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “slightly above” or “slightly below” the endpoint without affecting the desired result.

The use herein of the terms “including,” “comprising,” or “having,” and variations thereof, is meant to encompass the elements listed thereafter and equivalents thereof as well as additional elements. Embodiments recited as “including,” “comprising,” or “having” certain elements are also contemplated as “consisting essentially of”0 and “consisting of” those certain elements. As used herein, “and/or” refers to and encompasses any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations where interpreted in the alternative (“or”).

As used herein, the transitional phrase “consisting essentially of” (and grammatical variants) is to be interpreted as encompassing the recited materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention. See, In re Herz, 537 F.2d 549, 551-52, 190 U.S.P.Q. 461, 463 (CCPA 1976) (emphasis in the original); see also MPEP § 2111.03. Thus, the term “consisting essentially of” as used herein should not be interpreted as equivalent to “comprising.”

Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise-indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. For example, if a concentration range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc., are expressly enumerated in this specification. These are only examples of what is specifically intended, and all possible combinations of numerical values between and including the lowest value and the highest value enumerated are to be considered to be expressly stated in this disclosure.

I. Polynucleotides and Polypeptides

Provided in this disclosure are fusion proteins in which an N⁶-methyladenosine (m⁶A) binding domain of a YT521-B homology (YTH) domain-containing protein is fused to a catalytic domain of a cytidine deaminase or a catalytic domain of an adenosine deaminase. The fusion proteins bind to m⁶A-containing RNA and exhibit nucleotide deaminase activity, for example cytidine deaminase activity or adenosine deaminase activity. The fusion protein converts cytidine or uridine, or adenosine to inosine, in RNA molecules, thereby allowing detection of m⁶A residues in RNA molecules.

Provided herein is a fusion protein comprising an N⁶-methyladenosine (m⁶A) binding domain of a YT521-B homology (YTH) domain-containing protein fused to a catalytic domain of a cytidine deaminase or a catalytic domain of an adenosine deaminase.

As used throughout, a “fusion protein” is a protein comprising two different polypeptide sequences, i.e. a binding domain and a catalytic domain, that are joined or linked to form a single polypeptide. The two amino acid sequences are encoded by separate nucleic acid sequences that have been joined so that they are transcribed and translated to produce a single polypeptide. In some embodiments, the fusion protein comprises, in the following order, an m⁶A binding domain, and a catalytic domain of a cytidine deaminase or an adenosine deaminase.

As used throughout, “m⁶A” refers to posttranscriptional methylation of an adenosine residue in the RNA of prokaryotes and eukaryotes (e.g., mammals, insects, plants and yeast).

As used throughout, an m⁶A binding domain of a YT521-B homology (YTH) domain-containing protein is a polypeptide fragment of a YTH domain-containing protein that binds to m⁶A-containing RNA. The m⁶A binding domain derived from a YT521-B homology (YTH) domain-containing protein can be of any size as long as it retains binding activity and is not the full-length YTH domain-containing protein. In some embodiments, the binding domain retains at least about 75%, 80%, 90%, 95%, or 99% of the binding activity of the wildtype YTH domain-containing protein from which the binding domain is derived.

In some embodiments, the the m⁶A binding domain comprises a polypeptide having at least 95% identity, for example, at least about 95%, 96%, 97%, 98% or 99% identity, to SEQ ID NO: 1 (amino acid sequence of YTHDF2-YTH, an m⁶A binding domain of YTHDF2), SEQ ID NO: 2 (amino acid sequence of YTHDF2-YTH_W432A_W486A, a mutated m⁶A binding domain of YTHDF2), SEQ ID NO: 3 (amino acid sequence of YTHDF2-YTHmut, an amino acid sequence that includes theYTH domain of YTHDF2, and does not include the m⁶A-binding domain), SEQ ID NO: 4 (amino acid sequence of YTHDF2-YTHmut, an amino acid sequence comprising SEQ ID NO: 3, with a W432A mutation and a W486a mutation), SEQ ID NO: 5 (amino acid sequence of YTHDF2-YTH D422N, a mutated m⁶A binding domain of YTHDF2), SEQ ID NO: 6 (amino acid sequence of an m⁶A binding domain of YTHDF1), SEQ ID NO: 7 (amino acid sequence of YTHDF lmut, an amino acid sequence that includes theYTH domain of YTHDF2, and does not include the m⁶A-binding domain), SEQ ID NO: 8 (amino acid sequence of YTHDF1 D401N, a mutated m⁶A binding domain of YTHDF1), SEQ ID NO: 9 (amino acid sequence of an m⁶A binding domain of YTHDF3); SEQ ID NO: 10 (amino acid sequence of an m⁶A binding domain of YTHDC1) or SEQ ID NO: 11 (amino acid sequence of an m⁶A binding domain of YTHDC2).

As used throughout, a catalytic domain of a cytidine deaminase is a polypeptide comprising a cytidine deaminase, for example, Apolipoprotein B MRNA Editing Enzyme Catalytic Subunit (APOBEC1), activation induced cytidine deaminase (AICDA) or Apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3A (APOBEC3A), or a catalytic fragment thereof, that catalyzes deamination of cytidine (“C”) to uridine (“U”) in RNA molecules. As used throughout, a catalytic domain of an adenosine deaminase, is a polypeptide comprising an adenosine deaminase, for example, double-stranded RNA-specific adenosine deaminase (ADAR1), or a catalytic fragment thereof, that catalyzes deamination of adenosine (“A”) to inosine (“I”) in RNA molecules. In some embodiments, the catalytic domain retains at least about 75%, 80%, 90%, 95%, or 99% of the enzymatic activity of the wildtype deaminase from which the domain is derived.

In some embodiments, the catalytic domain comprises a polypeptide having at least 95% identity, for example, at least about 95%, 96%, 97%, 98% or 99% identity, to SEQ ID NO: 12 (amino acid sequence of rAPOBEC1) or its catalytic domain SEQ ID NO: 61, SEQ ID NO: 13 (amino acid sequence of hAICDA) or its catalytic domain (SEQ ID NO: 62); SEQ ID NO: 14 (amino acid sequence of hAPOBEC3A) or its catalytic domain (SEQ ID NO: 63); or SEQ ID NO: 15 (amino acid sequence of catalytic domain of ADAR1).

The catalytic domain can also comprise a polypeptide having at least 95% identity to SEQ ID NO: 64 (amino acid sequence of catalytic domain of AID-C12), as set forth in U.S. Patent Application Publication No. 20190010478.

In some embodiments, the m⁶A binding domain is fused to the catalytic domain via a peptide linker. The peptide linker can be about 2 to about 150 amino acids in length. For example, the linker can be a linker of from about 5 to about 20 amino acids in length, from about 5 to about 25 amino acids in length, from about 10 to about 30 amino acids in length, 5 to about 35 amino acids in length, from about 5 to about 40 amino acids in length, from about 5 to about 45 amino acids in length, from about 5 to about 50 amino acids in length, from about 5 to about 55 amino acids in length, from about 5 to about 60 amino acids in length, from about 5 to about 65 amino acids in length, from about 5 to about 70 amino acids in length, from about 5 to about 75 amino acids in length, from about 5 to about 80 amino acids in length, from about 5 to about 85 amino acids in length, from about 5 to about 90 amino acids in length, from about 5 to about 95 amino acids in length, from about 5 to about 100 amino acids in length, from about 5 to about 105 amino acids in length, from about 5 to about 110 amino acids in length, from about 5 to about 115 amino acids in length, from about 5 to about 120 amino acids in length, from about 5 to about 125 amino acids in length, from about 5 to about 130 amino acids in length, from about 5 to about 135 amino acids in length, from about 5 to about 140 amino acids in length, from about 5 to about 145 amino acids in length, or from about 5 to about 150 amino acids in length.

Exemplary peptide linkers include, but are not limited to, peptide linkers comprising SEQ ID NO: 16 (SGSETPGTSESATPE), SEQ ID NO: 17 (SGSETPGTSESATPES), SEQ ID NO: 18 ((GGGGS)₃), SEQ ID NO: 19 ((GGGGS)₁₀), SEQ ID NO: 20 ((GGGGS)₂₀), SEQ ID NO: 21 (A(EAAAK)₃A), SEQ ID NO: 22 (A(EAAAK)₁₀A), or SEQ ID NO: 23 (A(EAAAK)₂₀A).

In some embodiments, the fusion protein further comprises a localization element. In some embodiments, the localization element is fused to the N-terminus or the C-terminus of the fusion protein. As used herein, a localization element targets or localizes the fusion protein to one or more subcellular compartments. Subcellular compartments include but are not limited to, the nucleus, the endoplasmic reticulum, the mitochondria, chromatin, the cellular membrane, and RNA granules (for example, P-bodies, stress granules and transport granules). In some embodiments, the fusion protein can be targeted to the nuclear lamina, nuclear speckles nuclear paraspeckles in the nucleus of a cell. In some embodiments, the protein can be targeted to the outer mitochondrial membrane or the inner mitochondrial membrane.

Exemplary localization elements include, but are not limited to, a peptide comprising a nuclear localization signal, for example, SEQ ID NO: 24 (PKKKRKV), a peptide comprising a nuclear export signal, for example, SEQ ID NO: 25 (LPPLERLTL), a peptide comprising an endoplasmic reticulum targeting sequence, for example, SEQ ID NO: 26 (MDPVVVLGLCLSCLLLLSLWKQSYGGG), or SEQ ID NO: 60 (METDTLLLWVLLLWVPGSTGD), a peptide comprising a Myc tag, for example, SEQ ID NO: 27 (EQKLISEEDL), a peptide comprising a V5 tag, for example, SEQ ID NO: 28 (GKPIPNPLLGLDST) or SEQ ID NO: 29 (IPNPLLGLD), a peptide comprising a FLAG tag, for example, SEQ ID NO: 30 (DYKDDDDK), a peptide comprising a 3 X FLAG tag, for example, SEQ ID NO: 31 (DYKDHDGDYKDHDIDYKDDDDK) and a peptide comprising a DHFR destabilization domain, for example, SEQ ID NO: 32 (ISLIAALAVDHVIGMETVMPWNLPADLAWFKRNTLNKPVIMGRHTWESIGRPLPGRKNIILSSQPSTDDRVTWVKSVDEAIAACGDVPEIMVIGGGRVYEQFLPKAQKLYLTHIDAEVEGDTHFPDYEPDDWESVF SEFHDADAQNSHSYCFEILERR).

“Polypeptide,” “peptide,” and “protein” are used interchangeably herein to refer to a polymer of amino acid residues. As used herein, the terms encompass amino acid chains of any length, including full-length proteins, wherein the amino acid residues are linked by covalent peptide bonds.

Modifications to any of the polypeptides or proteins provided herein are made by known methods. By way of example, modifications are made by site specific mutagenesis of nucleotides in a nucleic acid encoding the polypeptide, thereby producing a DNA encoding the modification, and thereafter expressing the DNA in recombinant cell culture to produce the encoded polypeptide. Techniques for making substitution mutations at predetermined sites in DNA having a known sequence are well known. For example, M13 primer mutagenesis and PCR-based mutagenesis methods can be used to make one or more substitution mutations. Any of the nucleic acid sequences provided herein can be codon-optimized to alter, for example, maximize expression, in a host cell or organism. SEQ ID NOs: 58 and 59 are exemplary codon-optimized nucleic acids for expression and purification of APOBEC1-YTH and APOBEC1-YTHmut, respectively.

The amino acids in the polypeptides described herein can be any of the 20 naturally occurring amino acids, D-stereoisomers of the naturally occurring amino acids, unnatural amino acids and chemically modified amino acids. Unnatural amino acids (that is, those that are not naturally found in proteins) are also known in the art, as set forth in, for example, Zhang et al. “Protein engineering with unnatural amino acids,” Curr. Opin. Struct. Biol. 23(4): 581-587 (2013); Xie et al. “Adding amino acids to the genetic repertoire,” 9(6): 548-54 (2005)); and all references cited therein. B and γ amino acids are known in the art and are also contemplated herein as unnatural amino acids.

As used herein, a chemically modified amino acid refers to an amino acid whose side chain has been chemically modified. For example, a side chain can be modified to comprise a signaling moiety, such as a fluorophore or a radiolabel. A side chain can also be modified to comprise a new functional group, such as a thiol, carboxylic acid, or amino group. Post-translationally modified amino acids are also included in the definition of chemically modified amino acids.

Also contemplated are conservative amino acid substitutions. By way of example, conservative amino acid substitutions can be made in one or more of the amino acid residues, for example, in one or more lysine residues of any of the polypeptides provided herein. One of skill in the art would know that a conservative substitution is the replacement of one amino acid residue with another that is biologically and/or chemically similar. The following eight groups each contain amino acids that are conservative substitutions for one another:

1) Alanine (A), Glycine (G);

2) Aspartic acid (D), Glutamic acid (E);

3) Asparagine (N), Glutamine (Q);

4) Arginine (R), Lysine (K);

5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V);

6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W);

7) Serine (S), Threonine (T); and

8) Cysteine (C), Methionine (M).

By way of example, when an arginine to serine is mentioned, also contemplated is a conservative substitution for the serine (e.g., threonine). Nonconservative substitutions, for example, substituting a lysine with an asparagine, are also contemplated.

Any of the polypeptides described herein can further comprise a detectable moiety, for example, a fluorescent protein or fragment thereof. Examples of fluorescent proteins include, but are not limited to, yellow fluorescent protein (YFP, for example, Venus), green fluorescent protein (GFP), and red fluorescent protein (RFP) as well as derivatives, for example, mutant derivatives, of these proteins. See, for example, Chudakov et al. “Fluorescent Proteins and Their Applications in Imaging Living Cells and Tissues,” Physiological Reviews 90(3): 1103-1163 (2010); and Specht et al., “A Critical and Comparative Review of Fluorescent Tools for Live-Cell Imaging,” Annual Review of Physiology 79: 93-117 (2017))

Any of the polypeptides described herein can further comprise an affinity tag, for example a polyhistidine tag ((His)₆), albumin-binding protein, alkaline phosphatase, an AU1 epitope, an AU5 epitope, a biotin-carboxy carrier protein (BCCP) or a FLAG epitope, to name a few. See, Kimple et al. “Overview of Affinity Tags for Protein Purification, Curr. Protoc. Protein Sci. 73: Unit-9.9 (2013).

Recombinant nucleic acids encoding any of the polypeptides described herein are also provided. For example, a recombinant nucleic acid encoding a polypeptide that has at least 95%, for example, at least about 95%, 96%, 97%, 98% or 99%, identity to any one of SEQ ID NOs 1-32 is also provided.

As used throughout, the term “nucleic acid” or “nucleotide” refers to deoxyribonucleic acids (DNA) or ribonucleic acids (RNA) and polymers thereof in either single- or double-stranded form. It is understood that when an RNA is described, its corresponding cDNA is also described, wherein uridine is represented as thymidine. Unless specifically limited, the term encompasses nucleic acids containing known analogues of natural nucleotides that have similar properties as the reference nucleic acid and are metabolized in a manner similar to naturally occurring nucleotides. A nucleic acid sequence can comprise combinations of deoxyribonucleic acids and ribonucleic acids. Such deoxyribonucleic acids and ribonucleic acids include both naturally occurring molecules and synthetic analogues. The polynucleotides of the invention also encompass all forms of sequences including, but not limited to, single-stranded forms, double-stranded forms, hairpins, stem-and-loop structures, and the like.

As used throughout, RNA can be messenger RNA (mRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), a regulatory RNA, a transfer-messenger RNA (tmRNA), ribosomal RNA (rRNA), microRNA (miRNA), long noncoding RNA (lncRNA) or circular RNA (circRNA).

Unless otherwise indicated, a particular nucleic acid sequence also implicitly encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions), alleles, orthologs, SNPs, and complementary sequences as well as the sequence explicitly indicated. Specifically, degenerate codon substitutions may be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues (Batzer et al., Nucleic Acid Res. 19:5081 (1991); Ohtsuka et al., J. Biol. Chem. 260:2605-2608 (1985); and Rossolini et al., Mol. Cell. Probes 8:91-98 (1994)).

The term “ identity” or “substantial identity,” as used in the context of a polynucleotide or polypeptide sequence described herein, refers to a sequence that has at least 60% sequence identity to a reference sequence. Alternatively, percent identity can be any integer from 60% to 100%. Exemplary embodiments include at least: 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, as compared to a reference sequence using the programs described herein; preferably BLAST using standard parameters, as described below. One of skill will recognize that these values can be appropriately adjusted to determine corresponding identity of proteins encoded by two nucleotide sequences by taking into account codon degeneracy, amino acid similarity, reading frame positioning and the like.

For sequence comparison, typically one sequence acts as a reference sequence to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters can be used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities for the test sequences relative to the reference sequence, based on the program parameters.

A “comparison window,” as used herein, includes reference to a segment of any one of the number of contiguous positions selected from the group consisting of from 20 to 600, usually about 50 to about 200, more usually about 100 to about 150 in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well-known in the art. Optimal alignment of sequences for comparison may be conducted by the local homology algorithm of Smith and Waterman Add. APL. Math. 2:482 (1981), by the homology alignment algorithm of Needleman and Wunsch J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson and Lipman Proc. Natl. Acad. Sci. (U.S.A.) 85: 2444 (1988), by computerized implementations of these algorithms (e.g., BLAST), or by manual alignment and visual inspection.

Algorithms that are suitable for determining percent sequence identity and sequence similarity are the BLAST and BLAST 2.0 algorithms, which are described in Altschul et al. (1990) J. Mol. Biol. 215: 403-410 and Altschul et al. (1977) Nucleic Acids Res. 25: 3389-3402, respectively. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (NCBI) web site. The algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al, supra). These initial neighborhood word hits acts as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a word size (W) of 28, an expectation (E) of 10, M=1, N=−2, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a word size (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)).

The BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.01, more preferably less than about 10⁻⁵, and most preferably less than about 10⁻²⁰.

II. Constructs and Vectors

Also provided is a DNA construct comprising a promoter operably linked to a recombinant nucleic acid described herein. A nucleic acid is “operably linked” when it is placed into a functional relationship with another nucleic acid sequence. Numerous promoters can be used in the constructs described herein. A promoter is a region or a sequence located upstream and/or downstream from the start of transcription that is involved in recognition and binding of RNA polymerase and other proteins to initiate transcription. The promoter can be a eukaryotic or a prokaryotic promoter. In some embodiments the promoter is an inducible promoter. In some embodiments, the promoter is a constitutive promoter.

The recombinant nucleic acids provided herein can be included in expression cassettes for expression in a host cell or an organism of interest. The cassette will include 5′ and 3′ regulatory sequences operably linked to a recombinant nucleic acid provided herein that allows for expression of the modified polypeptide. The cassette may additionally contain at least one additional gene or genetic element to be cotransformed into the organism. Where additional genes or elements are included, the components are operably linked. Alternatively, the additional gene(s) or element(s) can be provided on multiple expression cassettes. Such an expression cassette is provided with a plurality of restriction sites and/or recombination sites for insertion of the polynucleotides to be under the transcriptional regulation of the regulatory regions. The expression cassette may additionally contain a selectable marker gene. The expression cassette will include in the 5′ to 3′ direction of transcription: a transcriptional and translational initiation region (i.e., a promoter), a polynucleotide of the invention, and a transcriptional and translational termination region (i.e., termination region) functional in the cell or organism of interest. The promoters of the invention are capable of directing or driving expression of a coding sequence in a host cell. The regulatory regions (i.e., promoters, transcriptional regulatory regions, and translational termination regions) may be endogenous or heterologous to the host cell or to each other. As used herein, “heterologous” in reference to a sequence is a sequence that originates from a foreign species, or, if from the same species, is substantially modified from its native form in composition and/or genomic locus by deliberate human intervention.

Additional regulatory signals include, but are not limited to, transcriptional initiation start sites, operators, activators, enhancers, other regulatory elements, ribosomal binding sites, an initiation codon, termination signals, and the like. See Sambrook et al. (1992) Molecular Cloning: A Laboratory Manual, ed. Maniatis et al. (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.) (hereinafter “Sambrook 11”); Davis et al., eds. (1980) Advanced Bacterial Genetics (Cold Spring Harbor Laboratory Press), Cold Spring Harbor, N.Y., and the references cited therein.

The expression cassette can also comprise a selectable marker gene for the selection of transformed cells. Marker genes include genes conferring antibiotic resistance, such as those conferring hygromycin resistance, ampicillin resistance, gentamicin resistance, neomycin resistance, to name a few. Additional selectable markers are known and any can be used.

In preparing the expression cassette, the various DNA fragments may be manipulated, so as to provide for the DNA sequences in the proper orientation and, as appropriate, in the proper reading frame. Toward this end, adapters or linkers may be employed to join the DNA fragments or other manipulations may be involved to provide for convenient restriction sites, removal of superfluous DNA, removal of restriction sites, or the like. For this purpose, in vitro mutagenesis, primer repair, restriction, annealing, resubstitutions, e.g., transitions and transversions, may be involved.

In preparing the expression cassette, the various DNA fragments may be manipulated, so as to provide for the DNA sequences in the proper orientation and, as appropriate, in the proper reading frame. Toward this end, adapters or linkers may be employed to join the DNA fragments or other manipulations may be involved to provide for convenient restriction sites, removal of superfluous DNA, removal of restriction sites, or the like. For this purpose, in vitro mutagenesis, primer repair, restriction, annealing, resubstitutions, e.g., transitions and transversions, may be used.

Further provided is a vector comprising a nucleic acid or expression cassette set forth herein. The vector is contemplated to have the necessary functional elements that direct and regulate transcription of the inserted nucleic acid. These functional elements include, but are not limited to, a promoter, regions upstream or downstream of the promoter, such as enhancers that may regulate the transcriptional activity of the promoter, an origin of replication, appropriate restriction sites to facilitate cloning of inserts adjacent to the promoter, antibiotic resistance genes or other markers which can serve to select for cells containing the vector or the vector containing the insert, RNA splice junctions, a transcription termination region, or any other region which may serve to facilitate the expression of the inserted gene or hybrid gene (See generally, Sambrook et al. Molecular Cloning: A Laboratory Manual, 4^(th) ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, 2012). The vector, for example, can be a plasmid.

There are numerous E. coli expression vectors known to one of ordinary skill in the art, which are useful for the expression of a nucleic acid. Other microbial hosts suitable for use include bacilli, such as Bacillus subtilis, and other enterobacteriaceae, such as Salmonella, Senatia, and various Pseudomonas species. In these prokaryotic hosts, one can also make expression vectors, which will typically contain expression control sequences compatible with the host cell (e.g., an origin of replication). In addition, any number of a variety of well-known promoters will be present, such as the lactose promoter system, a tryptophan (Trp) promoter system, a beta-lactamase promoter system, or a promoter system from phage lambda. Additionally, yeast expression can be used. Provided herein is a nucleic acid encoding a polypeptide of the present invention, wherein the nucleic acid can be expressed by a yeast cell. More specifically, the nucleic acid can be expressed by Pichia pastoris or S. cerevisiae.

Mammalian cells also permit the expression of proteins in an environment that favors important post-translational modifications such as folding and cysteine pairing, addition of complex carbohydrate structures, and secretion of active protein. Vectors useful for the expression of active proteins in mammalian cells are known in the art and can contain genes conferring hygromycin resistance, genticin or G418 resistance, or other genes or phenotypes suitable for use as selectable markers, or methotrexate resistance for gene amplification. A number of suitable host cell lines capable of secreting intact human proteins have been developed in the art, and include CHO cells, HeLa cells, COS-7 cells, myeloma cell lines, Jurkat cells, etc. Expression vectors for these cells can include expression control sequences, such as an origin of replication, a promoter, an enhancer, and necessary information processing sites, such as ribosome binding sites, RNA splice sites, polyadenylation sites, and transcriptional terminator sequences. Preferred expression control sequences are promoters derived from immunoglobulin genes, SV40, Adenovirus, Bovine Papilloma Virus, etc.

The expression vectors described herein can also include the nucleic acids as described herein under the control of an inducible promoter such as the tetracycline inducible promoter or a glucocorticoid inducible promoter. The nucleic acids of the present invention can also be under the control of a tissue-specific promoter to promote expression of the nucleic acid in specific cells, tissues or organs. Any regulatable promoter, such as a metallothionein promoter, a heat-shock promoter, and other regulatable promoters, of which many examples are well known in the art are also contemplated. Furthermore, a Cre-loxP inducible system can also be used, as well as a Flp recombinase inducible promoter system, both of which are known in the art.

Insect cells also permit the expression of the polypeptides. Recombinant proteins produced in insect cells with baculovirus vectors undergo post-translational modifications similar to that of wild-type mammalian proteins.

III. Cells and Transgenic Animals

As such, aspects of this disclosure include host cells and transgenic animals comprising the nucleic acid sequences described herein as well as methods of making such cells and transgenic animals.

A host cell comprising a nucleic acid or a vector described herein is provided. The host cell can be an in vitro, ex vivo, or in vivo host cell. Populations of any of the host cells described herein are also provided. A cell culture comprising one or more host cells described herein is also provided. Methods for the culture and production of many cells, including cells of bacterial (for example E. coli and other bacterial strains), animal (especially mammalian), and archebacterial origin are available in the art. See e.g., Sambrook, Ausubel, and Berger (all supra), as well as Freshney (1994) Culture of Animal Cells, a Manual of Basic Technique, 3^(rd) Ed., Wiley-Liss, New York and the references cited therein; Doyle and Griffiths (1997) Mammalian Cell Culture: Essential Techniques John Wiley and Sons, NY; Humason (1979) Animal Tissue Techniques, 4^(th) Ed. W.H. Freeman and Company; and Ricciardelli, et al., (1989) In vitro Cell Dev. Biol. 25:1016-1024.

The host cell can be a prokaryotic cell, including, for example, a bacterial cell. Alternatively, the cell can be a eukaryotic cell, for example, a mammalian cell. In some embodiments, the cell can be an HEK293T cell, a Chinese hamster ovary (CHO) cell, a COS-7 cell, a HELA cell, an avian cell, a myeloma cell, a Pichia cell, an insect cell or a plant cell. A number of other suitable host cell lines have been developed and include myeloma cell lines, fibroblast cell lines, and a variety of tumor cell lines such as melanoma cell lines. The vectors containing the nucleic acid segments of interest can be transferred or introduced into the host cell by well-known methods, which vary depending on the type of cellular host.

As used herein, the phrase “introducing” in the context of introducing a nucleic acid into a cell refers to the translocation of the nucleic acid sequence from outside a cell to inside the cell. In some cases, introducing refers to translocation of the nucleic acid from outside the cell to inside the nucleus of the cell. Various methods of such translocation are contemplated, including but not limited to, electroporation, nanoparticle delivery, viral delivery, contact with nanowires or nanotubes, receptor mediated internalization, translocation via cell penetrating peptides, liposome mediated translocation, DEAE dextran, lipofectamine, calcium phosphate or any method now known or identified in the future for introduction of nucleic acids into prokaryotic or eukaryotic cellular hosts. A targeted nuclease system (e.g., an RNA-guided nuclease, a transcription activator-like effector nuclease (TALEN), a zinc finger nuclease (ZFN), or a megaTAL (MT) (Li et al. Signal Transduction and Targeted Therapy 5, Article No. 1 (2020)) can also be used to introduce a nucleic acid, for example, a nucleic acid encoding a fusion protein described herein, into a host cell

The CRISPR/Cas9 system, an RNA-guided nuclease system that employs a Cas9 endonuclease, can be used to edit the genome of a host cell or organism. The “CRISPR/Cas” system refers to a widespread class of bacterial systems for defense against foreign nucleic acid. CRISPR/Cas systems are found in a wide range of eubacterial and archaeal organisms. CRISPR/Cas systems include type I, II, and III sub-types. Wild-type type II CRISPR/Cas systems utilize an RNA-mediated nuclease, for example, Cas9, in complex with guide and activating RNA to recognize and cleave foreign nucleic acid. Guide RNAs having the activity of both a guide RNA and an activating RNA are also known in the art. In some cases, such dual activity guide RNAs are referred to as a single guide RNA (sgRNA).

As used herein, the term “Cas9” refers to an RNA-mediated nuclease (e.g., of bacterial or archeal orgin, or derived therefrom). Exemplary RNA-mediated nucleases include the foregoing Cas9 proteins and homologs thereof. Other RNA-mediated nucleases include Cpf1 (See, e.g., Zetsche et al., Cell, Volume 163, Issue 3, p759-771, 22 Oct. 2015) and homologs thereof

Cas9 homologs are found in a wide variety of eubacteria, including, but not limited to bacteria of the following taxonomic groups: Actinobacteria, Aquificae, Bacteroidetes-Chlorobi, Chlamydiae-Verrucomicrobia, Chlroflexi, Cyanobacteria, Firmicutes, Proteobacteria, Spirochaetes, and Thermotogae. An exemplary Cas9 protein is the Streptococcus pyogenes Cas9 protein. Additional Cas9 proteins and homologs thereof are described in, e.g., Chylinksi, et al., RNA Biol. 2013 May 1; 10(5): 726-737 ; Nat. Rev. Microbiol. 2011 June; 9(6): 467-477; Hou, et al., Proc Natl Acad Sci U S A. 2013 Sep. 24;110(39):15644-9; Sampson et al., Nature. 2013 May 9;497(7448):254-7; and Jinek, et al., Science. 2012 Aug. 17;337(6096):816-21. Variants of any of the Cas9 nucleases provided herein can be optimized for efficient activity or enhanced stability in the host cell. Thus, engineered Cas9 nucleases are also contemplated. See, for example, “Slaymaker et al., “Rationally engineered Cas9 nucleases with improved specificity,” Science 351 (6268): 84-88 (2016)).

Any of the fusion proteins described herein can be purified or isolated from a host cell or population of host cells. For example, a recombinant nucleic acid encoding any of the fusion proteins described herein can be introduced into a host cell under conditions that allow expression of the fusion protein. In some embodiments, the recombinant nucleic acid is codon-optimized for expression. After expression in the host cell, the fusion protein can be isolated or purified, for example, as described in the Examples, or using other purification methods known in the art. As used herein, the term “isolated” or “purified” means that the protein is substantially free of other components found in the cell.

Also provided is a non-human transgenic animal comprising a mammalian host cell that comprises a nucleic acid or a vector described herein described. Methods for making transgenic animals, include, but are not limited to, oocyte pronuclear DNA microinjection, intracytoplasmic sperm injection, embryonic stem cell manipulation, somatic nuclear transfer, recombinase systems (for example, Cre-LoxP systems, Flp-FRT systems and others), zinc finger nucleases (ZNFs), transcriptional activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeat/CRISPR-associated protein 9 (CRISPR/Cas9). See, for example, Volobueva et al. Braz. J. Med. Biol. Res. 52(5): e8108 (2019)).

The term “transgenic animal” as used herein means an animal into which a genetic modification has been introduced by a genetic engineering procedure and in particular an animal into which has been introduced an exogenous nucleic add. That is the animal comprises a nucleic acid sequence encoding a fusion protein described herein, which is not normally present in the animal. Included are both progenitor and progeny animals. Progeny animals include animals which are descended from the progenitor as a result of sexual reproduction or cloning and which have inherited genetic material from the progenitor. Thus, the progeny animals comprise the genetic modification introduced into the parent. A transgenic animal may be developed, for example, from embryonic cells into which the genetic modification (e.g. exogenous nucleic acid sequence) has been directly introduced or from the progeny of such cells. The exogenous nucleic acid is introduced artificially into the animal (e.g. into a founder animal). Animals that are produced by transfer of an exogenous nucleic acid through breeding of the animal comprising the nucleic acid (into whom the nucleic acid was artificially introduced), which are progeny animals, are also included. Representative examples of non-human mammals include, but are not limited to non-human primates, mice, rats, rabbits, pigs, goats, sheep, horses, zebrafish and cows. A cell or a population of cells from any of the non-human transgenic animals provided herein is also provided.

The exogenous nucleic acid may be integrated into the genome of the animal or it may be present in an non-integrated form, e.g. as an autonomously-replicating unit, for example an artificial chromosome which does not integrate into the genome, but which is maintained and inherited substantially stably in the animal. the expression of a reporter protein is tissue-specific is contemplated for this invention. In some embodiments, the exogenous nucleic acid is under the control of a cell-specific or tissue-specific promoter. For example, transgenic animals that express a a fusion protein in specific cells or tissues can be produced by introducing a nucleic acid into fertilized eggs, embryonic stem cells or the germline of the animal, wherein the nucleic acid is under the control of a specific promoter which allows expression of the nucleic acid in specific types of cells or tissues. As used herein, a protein is expressed predominantly in a given tissue, cell type, cell lineage or cell, when 90% or greater of the observed expression occurs in the given tissue cell type, cell lineage or cell.

In some embodiments, the exogenous nucleic acid is under the control of an inducible promoter. Inducible promoter systems that can be used include the heat shock promoter (Halloran et al., Development 127 (9): 1953-60 (2000)), the tetracycline inducible system, the RU486/mifepristone inducible system and the ecdysone inducible system (Rossi and Blau, Curr. Opin. Biotech. 9: 451-456 0998)), to name a few. Inducible systems can also be used to allow expression of the fusion protein at designated times during development, expanding the temporal specificity of fusion protein expression in the transgenic animal.

IV. Methods

This disclosure also provides methods for detecting m⁶A residues in cellular RNA by using DART-Seq (deamination adjacent to RNA modification targets). The methods according to the present disclosure substantially improve the time and cost associated with global m⁶A detection and enables transcriptome-wide mapping of m⁶A in limited RNA samples, without the need for the use of m⁶A antibodies.

Provided herein is a method for detecting m⁶A residues in the RNA produced by one or more cells comprising: (a) isolating RNA from one or more cells that express a fusion protein described herein; (b) amplifying one or more target sequences in the isolated RNA; and (c) sequencing the one or more target sequences to identify cytidine to uridine deamination at sites adjacent to m⁶A residues, thus detecting the m⁶A residues in the RNA of the one of more cells. In some embodiments, the recombinant nucleic acid encoding the fusion protein is introduced into the one or more cells prior to isolating RNA from the cell. In some embodiments, the fusion protein is stably or transiently expressed in the cell, as described above.

In any of the methods provided herein, the RNA isolated from the one or more cells can be one or more RNAs selected from the group consisting of messenger RNA (mRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), a regulatory RNA, a transfer-messenger RNA (tmRNA), ribosomal RNA (rRNA), microRNA (miRNA), long noncoding RNA (lncRNA) or circular RNA (circRNA). In some embodiments, the methods further comprise enriching the isolated RNA for one or more RNAs of interest. For example, and not to be limiting, the isolated RNA be enriched for mRNA by removing rRNA from the isolated RNA.

As used herein, the term “adjacent” refers to a site of C to U conversion, or A to I conversion, that is immediately next to an m⁶A residue, as well as a site that is near an m⁶A residue, for example, from about 1 to about 20 nucleotides upstream or downstream of an m⁶A residue. For example, about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 nucleotides upstreatm or downstream of an m⁶A residue. Since C to U, or A to I conversions occur adjacent to m⁶A residues, the number and location of m⁶A residues in one or more RNA molecules of interest can be identified.

In some embodiments, m⁶A residues are detected in the transcriptome of one or more cells. As used herein, the term “transcriptome” refers to the set of all RNA transcripts, including coding and non-coding transcripts in a single cell or a population of cells. In some embodiments, the transcriptome of one or more cells can be analyzed under different conditions, for example, exposure of the one or more cells to a drug or a chemical.

It is understood that in any of the methods provided herein, a fusion protein that catalyzes cytidine to uridine deamination can be replaced with a fusion protein that catalyzes adenosine to inosine deamination to detect m⁶A residues in the RNA of the one of more cells.

In some embodiments, the RNA is isolated from a population of cells. In some embodiments, a population of cells is separated into individual compartments, for example, tissue culture wells, prior to isolation of RNA from single cells. In some embodiments the amount of isolated RNA used in the method is less than about 200 ng, 175 ng, 150 ng, 125 ng, 100 ng, 75 ng, 50 ng, 25 ng, 15 ng, 10 ng, 5 ng, 0.5 ng, 0.1 ng or 0.01 ng.

In any of the methods provided herein, the one or more cells can be prokaryotic or eukaryotic cells. In some embodiments, the eukaryotic cell is a mammalian cell, a plant cell or a yeast cell. In some embodiments, the cell is a primary cell. As used herein, the term “primary” in the context of a primary cell, or example, a primary stem cell refers to a cell that has not been transformed or immortalized. Such primary cells can be cultured, sub-cultured, or passaged a limited number of times (e.g., cultured 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 times). In some cases, the primary cells are adapted to in vitro culture conditions. In some cases, the primary cells are isolated from an organism, system, organ, or tissue, optionally sorted, and utilized directly without culturing or sub-culturing. In some cases, the primary cells are stimulated, activated, or differentiated. In some embodiments, the primary cells are neurons, brain cells or hematopoietic cells. In any of the methods described herein, the cell can be an in vitro, an ex vivo, or an in vivo cell.

In any of the methods described herein, the one or more target sequences can be amplified, for example, using reverse-transcriptase PCT (RT-PCR or RT-qPCR), to generate a cDNA that can be sequenced. In some embodiments, RNA-Seq is used for amplification and sequencing. In some embodiments, RNA-Seq is used for single cell sequencing or in situ sequencing of fixed tissue. See, Chu et al. “RNA sequencing: platform selection, experimental design, and data interpretation”. Nucleic Acid Therapeutics. 22 (4): 271-4 (2012); and Lee et al. “Highly multiplexed subcellular RNA sequencing in situ”. Science. 343 (6177): 1360-3 (2014). In some embodiments, targeted RNA-Seq is used for selecting and sequencing specific RNAs of interest. See, for example, Martin et al. “Targeted RNA Sequencing Assay to Characterize Gene Expression and Genomic Alterations,” J. Vis. Exp. 114: 54090 (2016).

Other sequencing methods that can be used to identify cytidine to uridine (thymidine in cDNA), or adenosine to inosine conversions include, but are not limited to, shotgun sequencing, bridge PCR, Sanger sequencing (including microfluidic Sanger sequencing), pyrosequencing, massively parallel signature sequencing, nanopore DNA sequencing, single molecule real-time sequencing (SMRT) (Pacific Biosciences, Menlo Park, Calif.), ion semiconductor sequencing, ligation sequencing, sequencing by synthesis (Illumina, San Diego, Calif.), Polony sequencing, 454 sequencing, solid phase sequencing, DNA nanoball sequencing, heliscope single molecule sequencing, mass spectroscopy sequencing, pyrosequencing, Supported Oligo Ligation Detection (SOLiD) sequencing, DNA microarray sequencing, RNAP sequencing, tunneling currents DNA sequencing, and any other DNA sequencing method identified in the future. One or more of the sequencing methods described herein can be used in high throughput sequencing methods. As used herein, the term “high throughput sequencing” refers to all methods related to sequencing nucleic acids where more than one nucleic acid sequence is sequenced at a given time.

Also provided is a method for detecting m⁶A residues in the RNA produced by one or more cells comprising: (a) fixing a cell or tissue that expresses a fusion protein described herein; and (b) detecting cytidine to uridine deamination in the RNA, wherein cytidine to uridine deamination is detected via mutation-sensitive in situ hybridization. See, for example, Baker et al. “Robust RNA-based in situ mutation detection delineates colorectal cancer subclonal evolution,” Nature Communications 8, Article number 1998 (2017). In some embodiments, the fusion protein is introduced into the cell under conditions that allow expression of the fusion protein prior to fixing the cell. In some embodiments, the cell or tissue is from an organism, for example, a transgenic animal described herein. In some embodiments, the cell or tissue is from a frozen sample, a formalin fixed sample or a paraffin-embedded specimen. In some embodiments, the cell or tissue is fixed and permeabilized. Methods for fixing and permeabilizing cells and tissues are known in the art. See, for example, Fernandez and Fuentes, “Fixation/Permeabilization: New Alternative Procedure for Immunofluorescence and mRNA In Situ Hybridization of Vertebrate and Invertebrate Embryos,” Developmental Dynamics 242: 503-517 (2013).

Further provided is a method for detecting m⁶A residues in a biological sample comprising:(a) isolating RNA from a biological sample; (b) contacting the RNA with a fusion protein described herein; (c) amplifying one or more target sequences in the RNA; and (d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues in the one or more target sequences, thus detecting the m⁶A residues in the one or more target sequences.

As used herein, a biological sample is a sample derived from a subject and includes, but is not limited to, any cell, tissue, or biological fluid. The sample can be, but is not limited to, blood, plasma, serum, sputum, hair, cheek tissue, urine, saliva, bronchoalveolar lavage fluids, biopsy (e.g. tissue or cells isolated from organ tissue, for example, from lung, liver, kidney, skin etc.), vaginal secretion, nasal secretion, skin, gastric secretion, or bone marrow specimens.

As used throughout, by subject is meant an individual. The subject can be an adult subject or a pediatric subject. Pediatric subjects include subjects ranging in age from birth to eighteen years of age. Preferably, the subject is an animal, for example, a mammal such as a primate, and, more preferably, a human. Non-human primates are subjects as well. The term subject includes domesticated animals, such as cats, dogs, etc., livestock (for example, cattle, horses, pigs, sheep, goats, etc.) and laboratory animals (for example, ferret, chinchilla, mouse, rabbit, rat, gerbil, guinea pig, etc.). Thus, veterinary uses and medical formulations are contemplated herein.

Also provided is method for diagnosing a disease in a subject comprising: (a) isolating RNA from a biological sample; (b) contacting the RNA with a fusion protein described herein; (c) amplifying one or more target sequences in the RNA; and (d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues in the one or more target sequences to identify a pattern of m⁶A residues, a difference in the pattern of m⁶A residues as compared to a reference pattern of m⁶A residues from a reference population(s), indicating the subject has or is at risk for developing the disease. As used throughout, a reference pattern, i.e., a control pattern, provides the number and/or location of m⁶A residues in one or more target sequences that can be used for comparison of the pattern obtained from a biological sample. In some embodiments, the reference pattern of m⁶A residues in the one or more target sequences is from a subject or population(s) that does not have the disease. In some embodiments, the reference pattern of m⁶A residues in the one or more target sequences is from subject or a reference population that has been successfully treated for the disease. In some embodiments, the reference pattern of m⁶A residues in the one or more target sequences is from subject or a reference population that has a particular stage of the disease. In the methods set forth herein, the difference in the pattern of m⁶A residues can be an increase or a decrease in m⁶A residues. The difference in the pattern of m⁶A residues can also be a change in the presence or absence of m⁶A residues at different locations in the RNA. For example, methylation may occur at one or more adenosine residues in the one or more target sequences at different locations as compared to the reference pattern. In another example, methylation may not occur at one or more adenosine residues in one or more target sequences as compared to the reference pattern.

Further provided is a method for determining the efficacy of a selected treatment for a disease in a subject comprising: (a) isolating RNA from a biological sample from the subject before the selected treatment; (b) contacting the RNA with the fusion protein described herein, (c) amplifying one or more target sequences in the RNA; (d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues to identify a pattern of m⁶A residues in the one or more target sequences; (e) treating the subject with the selected treatment, (f) isolating RNA from a biological sample from the subject at one or more times after administration of the selected treatment; (g) amplifying one or more target sequences in the RNA of step f); (h) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues to identify a pattern of m⁶A residues in the one or more target sequences of step g); and (i) comparing the pattern of m⁶A residues identified in step (d) and (h) to determine whether the pattern of m⁶A residues is the same or whether the m⁶A residues detected in step (d) or (h) is more similar to a reference pattern, a pattern of m⁶A residues in step (h) more similar to the reference pattern indicating the selected treatment is effective for treating the disease in the subject.

In some embodiments, the reference pattern of m⁶A residues in the one or more target sequences is from a subject or population(s) that does not have the disease. In some embodiments, the reference pattern of m⁶A residues in the one or more target sequences is from subject or a reference population that has been successfully treated for the disease. In some embodiments, the reference pattern of m⁶A residues in the one or more target sequences is from subject or a reference population that has a particular stage of the disease that is indicative of improvement.

In the methods set forth herein, the similarity in the pattern of m⁶A residues can be a similarity in the number of m⁶A residues in the one or more target sequences as compared to a reference pattern. The similarity in the pattern of m⁶A residues can also be a similarity in the presence or absence of m⁶A residues at different locations in the RNA. For example, methylation may occur at one or more adenosine residues in the one or more target sequences, at the same location(s), as compared to the reference pattern. In another example, methylation may not occur at one or more adenosine residues in one or more target sequences, at the same location(s), as compared to the reference pattern.

In some embodiments, treatment comprises one or more therapies selected from the group consisting of surgery, radiation, drug(s), chemotherapy, hormone therapy, immunotherapy, targeted therapy, antiviral therapy, and a stem cell transplant.

In some embodiments, RNA is isolated about 8 hours, 12 hours, 24 hours, 48 hours, one week, two weeks, three weeks, four weeks, two months, three months, four months, five months, six months, one year, or two years after treatment. By monitoring the subject after treatment, an improvement as well as a relapse or flare of the disease can be detected. For example, if two years after successful treatment, the pattern of m⁶A residues from the subject is more similar to a reference pattern indicating disease or worsening of disease, this is indicative of a relapse.

In some embodiments, the disease is selected from the group consisting of cancer, an autoimmune disorder, a neurodegenerative disorder and a viral infection.

As used herein, cancer is a disease characterized by the rapid and uncontrolled growth of aberrant cells. Cancer cells can spread locally or through the bloodstream and lymphatic system to other parts of the body. In some embodiments, the cancer is a solid tumor. In some embodiments, the cancer is a blood or hematological cancer. Exemplary cancers include, but are not limited to, breast cancer, prostate cancer, ovarian cancer, glioblastoma, cervical cancer, skin cancer, pancreatic cancer, colorectal cancer, bladder cancer, endometrial cancer, renal cancer, liver cancer, brain cancer, lymphoma, leukemia (for example, acute myeloid leukemia), myeloma, lung cancer, and the like.

As used herein, an autoimmune disease is a disease where the immune system cannot differentiate between a subject's own cells and foreign cells, thus causing the immune system to mistakenly attack healthy cells in the body. Examples of autoimmune disorders include, but are not limited to, inflammatory bowel disease, multiple sclerosis, psoriasis, rheumatoid arthritis, systemic lupus erythematosus, Graves' disease, type 1 diabetes, Sjogren's syndrome, autoimmune thyroid disease, and celiac disease.

As used herein, a neurodegenerative disease is a disease characterized by progressive dying and loss of neurons in the central nervous system. Examples of neurodegenerative diseases include, but are not limited to, Alzheimer's disease, vascular dementia, Lewy body dementia, amyotrophic lateral sclerosis, Parkinson's disease, prion disease, motor neuron diseases, Huntington's disease, spinocerebellar ataxia, and spinal muscular dystrophy.

Examples of viral infections include, but are not limited to, infections caused by DNA and RNA viruses. Examples include, but are not limited to, human immunodeficiency virus I (HIV 1), influenza A, herpes virus, hepatitis B, hepatitis C, human papillomavirus, Zika virus, Simian Virus 40, Epstein-Barr virus, Rous sarcoma virus, adenovirus, severe acute respiratory syndrome (for example, SARS-CoV-2, the causative agent of COVID-19), and Middle East respiratory syndrome (MERS).

V. Kits

Also provided is a kit for the detection of RNA modifications in a cell or tissue. The kit comprises: (a) one or more fusion proteins described herein; and (b) one or more primers for amplification of one or more target RNA sequences. Instructions for use of the kit can also be included. In some embodiments, the components of the kit, for example, the one or more fusion proteins and/or the primers, can be in a composition.

Compositions comprising a fusion protein or primers for amplification of one or more target sequences are also provided. The compositions may further comprise a diluent, solubilizer and/or an emulsifier, to be used with the methods disclosed herein. Once the composition has been formulated, it can be stored in a container, for example, a sealed vial, as a solution, suspension, gel, emulsion, solid, or as a dehydrated or lyophilized powder. In certain embodiments, such formulations can be stored either in a ready-to-use form or in a form (e.g., lyophilized) that is reconstituted prior to use.

In some embodiments, the kit further comprises any reagents required for performing one-step or two-step RT-PCR, including but not limited to amplification buffers, nucleases, and dNTPs, including deoxyadenosine dATP, dCTP, dGTP, and dTTP.

Disclosed are materials, compositions, and components that can be used for, can be used in conjunction with, can be used in preparation for, or are products of the disclosed methods and compositions. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these materials are disclosed that while specific reference of each various individual and collective combinations and permutations of these compounds may not be explicitly disclosed, each is specifically contemplated and described herein. For example, if a method is disclosed and discussed and a number of modifications that can be made to a number of molecules including in the method are discussed, each and every combination and permutation of the method, and the modifications that are possible are specifically contemplated unless specifically indicated to the contrary. Likewise, any subset or combination of these is also specifically contemplated and disclosed. This concept applies to all aspects of this disclosure including, but not limited to, steps in methods using the disclosed compositions. Thus, if there are a variety of additional steps that can be performed, it is understood that each of these additional steps can be performed with any specific method steps or combination of method steps of the disclosed methods, and that each such combination or subset of combinations is specifically contemplated and should be considered disclosed.

Publications cited herein and the material for which they are cited are hereby specifically incorporated by reference in their entireties.

EXAMPLES Example 1. APOBEC1-YTH constructs for m⁶A detection

A. Materials and Methods

It was reasoned that a strategy which alters the sequence near methylation sites would enable m⁶A detection by RNA-seq and thus overcome the major limitations of current methods. The preferred consensus sequence for m⁶A contains an invariable cytidine residue immediately following the m⁶A site (Rm⁶ACH, where R=A or G; H=A, C, or U). APOBEC1 is a cytidine deaminase which targets DNA and RNA to induce cytidine to uridine (C to U) editing (Navaratnam, et al. The p27 catalytic subunit of the apolipoprotein B mRNA editing enzyme is a cytidine deaminase. J Blot Chem 268, 20709-20712 (1993)). Thus, it was reasoned that recruitment of APOBEC1 to m⁶A sites would enable deamination of the cytidine immediately following m⁶A residues. A novel approach was used to edit m⁶A-adjacent cytidines in RNAs by fusing APOBEC1 to the m⁶A-binding YTH domain and detecting subsequent editing events with RNA-Seq.

The following examples present the DART-Seq (deamination adjacent to RNA modification targets) approach for detecting m⁶A sites. The utility of this approach is demonstrated for detecting m⁶A sites in cellular RNAs using transcriptome-wide mapping with as little as 10 nanograms of total RNA as input. This strategy provides new insights into clustering of m⁶A residues within individual transcript isoforms. This approach substantially improves the time and cost associated with global m⁶A detection and will enable transcriptome-wide mapping in limited RNA samples.

APOBEC1-YTH Constructs. APOBEC1-YTH Constructs were generated by fusing APOBEC1 to the m⁶A-binding YTH domain of YTHDF2 (Wang et al. N6-methyladenosine-dependent regulation of messenger RNA stability. Nature 505, 117-120 (2014); and Schwartz et al. Perturbation of m6A Writers Reveals Two Distinct Classes of mRNA Methylation at Internal and 5′ Sites. Cell Rep 8, 284-296 (2014)). (FIG. 1). YTH-HA was synthesized as a gene fragment (IDT), and YTH-HA or YTH^(mut) -HA were subsequently amplified using the YTH Fwd/YTH-HA Rev or YTH^(mut) Fwd/YTH-HA Rev primers (below). The YTH-HA sequence comprised amino acids 385-579 of human YTHDF2 fused at its C-terminal end to the HA tag (YPYDVPDYA)(SEQ ID NO: 33). The YTH^(mut)-HA fusion lacked amino acids 385-409 comprising the m⁶A-binding region. These YTH-HA fusions were then inserted downstream of the rat APOBEC1 editing domain (APOBEC1) in the pCMV-APOBEC1 plasmid (Addgene plasmid #73019, Watertown, Mass.) using the Xmal and Pmel restriction sites. A 15 amino acid linker was present between the APOBEC1 domain and the YTH domain.

Antibodies. The following antibodies and concentrations were used: rabbit anti-HA (Cell Signaling (Danvers, Mass.); 3724S; 1:1000), rabbit anti-m⁶A (Abcam (Cambridge, UK); ab151230; 1:1000), HRP-conjugated goat anti-rabbit (Abcam; ab6721; 1:2500), HRP-conjugated sheep anti-mouse (GE Healthcare (Chicago, Ill.); 95017-554; 1:2500), mouse anti-β-actin (Genscript (Piscataway, N.J.); A00702; 1:5000), rabbit anti-METTL3 (Abcam; ab195352; 1:1000), rabbit anti-cleaved caspase 3 (Proteintech (Rosemont, Ill.); 25546-1-AP, 1:1000), AlexaFluor 488-conjugated goat anti-rabbit (Thermo-Fisher (Waltham, Mass.); A-21206; 1:1000).

Cells. HEK293T cells from the American Type Culture Collection (ATCC) were cultured at 37° C. using DMEM supplemented with 10% FBS. METTL3-depleted cell lines were generated by cloning a METTL3-targeting sgRNA sequence (5′-GGAGTTGATTGAGGTAAAGCG-3′)(SEQ ID NO: 34) into the pSpCas9(BB)-2A-Puro (PX459) V2.0 plasmid (Addgene plasmid #62988). Plasmids were then transfected into HEK293T cells and stable cells were selected with puromycin. Validation of METTL3 depletion was performed with Western blot, RNA-Seq, and m⁶A immunoblotting. Camptothecin treatment was carried out for 5 hours at 37° C. using 6 μM final concentration of camptothecin (Sigma (St. Louis, Mo.)) from a 3 mM stock prepared in DMSO. Control cells were treated with the same volume of DMSO.

Cell viability measurements. HEK293T cells were transfected with APOBEC1-YTH and incubated for 24 hours at 37° C. Viability was assessed by trypan blue staining and manual counting of the proportion of viable cells compared to untransfected cells.

Differential gene expression analysis. Gene expression analysis was carried out using the deseq2 package (Love et al. Genome Biol. 15, 550 (2014)). mRNAs identified as being at least 2-fold increased/decreased in APOBEC1-YTH expressing cells relative to untransfected cells with a corrected p value <0.05 were reported.

Immunofluorescence. HEK293T cells were transfected with APOBEC1-YTH and fixed 24 h later using 4% paraformaldehyde. Cells were then permeabilized in 0.1% Triton X-100 in PBS and blocked in 1% BSA/PBS for 15 min at 25° C. Rabbit anti-HA antibody was added overnight at 4° C. After 3×5 min washes in 1X PBS, secondary antibody (AlexaFluor488-conjugated goat anti-rabbit (Thermo Fisher), 1:1000) was then added for 1 h at 25° C. Cells were washed again in 1X PBS and incubated in DAPI solution (1:10000 in PBS) for 2 min. Images were acquired on a Leica DMi8 inverted fluorescence microscope.

m⁶A immunoblotting. Equal amounts of total RNA were separated by agarose gel electrophoresis for 1 h at 70V. RNA was then transferred to a Hybond nylon membrane for 2 h using downward transfer with Ambion NorthernMax/Gly transfer buffer. Membranes were then crosslinked using a handheld UV lamp for 1 min with 254 nm light. Membranes were blocked for ≥30 min with 5% nonfat dry milk in 0.1% PBST and probed with anti-m⁶A antibody overnight at 4° C. Secondary antibody (HRP-goat anti-rabbit; 1:2500) was added for 1 h in blocking buffer and blots were developed with ECL (Amersham ECL Prime (Little Chalfont, UK)) and imaged using the BioRad Chemidoc imaging system.

m⁶A immunoprecipitation (MeRIP). 30 μg of total RNA was fragmented using Ambion 10× fragmentation reagent at 70° C. for 7 min. 1.5 ul of the fragmentation reaction was saved as input. The remainder was subjected to m⁶A immunoprecipitation by first coupling 12 μl of m⁶A antibody to 50 μl of Protein A/G magnetic beads (Pierce (Dallas, Tex.)) in 300 μl of IP buffer (10 mM Sodium Phosphate, 0.05% Triton-X 100, 140 mM NaCl) for 2 h at 4° C., rotating. Beads were then washed three times in IP buffer, and RNA was denatured for 5 min at 75° C. followed by 2-3 minute incubation on ice. RNA was then coupled with antibody-bound beads in 300 μl IP buffer for 2 h at 4° C., rotating. Beads were washed five times in 500 μl IP buffer and eluted in 300 μl elution buffer (5 mM Tris-HCl, pH 7.5, 1 mM EDTA, pH 8.0, 0.05% SDS, 4.2 μl Proteinase K (20 mg/ml; Thermo)) for 1.5 h at 50° C. while mixing. RNA was then collected with phenol:chloroform extraction and ethanol precipitation.

RT-qPCR. RNA was reverse transcribed using Superscript III with random hexamers according to the manufacturer's instructions (Thermo). cDNA was then used for quantitative PCR using the indicated primers and iQ SYBR Green Super Mix (Bio-Rad) in an Eppendorf RealPlex thermocycler. RNA levels were determined using the AACt method and were normalized to GAPDH levels (MeRIP-RT-qPCR) or ACTB levels (mRNA abundance).

Western blotting. Protein was loaded in a NuPAGE 4-12% Bis-Tris precast gel (Thermo) and separated at 180V. Transfers were carried out at 105V for 90 min to a Hybond PVDF membrane. Blocking was carried out for ≥30 min in 5% nonfat dry milk/0.1% PBST and antibodies were added overnight in 0.1% PBST at 4° C. Secondary antibodies were incubated on membranes in blocking buffer for 1 h at room temperature. ECL reagent (Amersham ECL Prime) was mixed 1:1 and added to the membranes, which were imaged using the BioRad Chemidoc imaging system.

RNA pulldown assays. RNA pulldowns were performed as previously described⁴. Briefly, 5 μg of bait RNA which contained a single A or m⁶A residue (5′- biotin-GUUCUUCUGUGGACUGUG-3′)(SEQ ID NO: 35) was bound to pre-washed streptavidin agarose beads (Sigma-Aldrich (St. Louis, MO)) in 200 μl binding buffer (10 mM Tris-HCl, pH 7.5, 1.5 mM MgCl₂, 150 mM KCl, 0.5 mM DTT, 0.05% (v/v) NP-40) at 4° C. for 1 h on a rotator. Beads were then washed twice with 0.5 mL binding buffer. Protein lysates were isolated from HEK293T cells transfected with APOBEC1-YTH or APOBEC1-YTH^(mut) for 24 h by adding lysis buffer (10 mM NaCl, 2 mM EDTA, 0.5% Triton X-100, 0.5 mM DTT, 10 mM Tris-HCl, pH 7.5, complete mammalian protease inhibitor cocktail (Sigma) and phosphatase inhibitor cocktail 2 (Sigma)). Cells were lysed by 30 strokes of dounce homogenization and then centrifuged at 10,000×g at 4° C. for 15 min. Supernatants were collected and pre-cleared by mixing with streptavidin agarose beads at 4° C. for 1 hour on a rotator. Beads were pelleted at 6,500 rpm for 1 min and supernatants were then mixed with binding buffer. Approximately 1 mg of lysate was mixed with 10 μl of RNasin (Promega) and then added to RNA-bound beads for 30 minutes at room temperature, followed by a 2 h incubation at 4° C. on a rotator. Beads were washed five times (6,500 rpm, 4° C.) in binding buffer and proteins were eluted (60° C., 850 rpm for 30 minutes in a thermomixer) using elution buffer (50 mM Tris-HCl, pH 8.0, 200 mM NaCl, 2% SDS, 1 mM biotin). Samples were spun down at 6,500 rpm for lmin at room temperature, and eluates were collected and mixed 1:1 with 2X NuPAGE sample buffer containing 2.5% μ-mercaptoethanol and analyzed via western blot.

In vitro transcription. RNA was synthesized using the HiScribe T7 in vitro transcription kit (New England Biolabs). 1 μg of purified PCR product was used for each reaction. Transcription was carried out overnight at 42° C. using either ATP or N⁶-meATP (Trilink (San Diego, Calif.). For experiments using varying amounts of m⁶A within an RNA, transcripts synthesized using all ATP or all N⁶-meATP were mixed in the indicated proportions.

In vitro deaminase assays. APOBEC1-YTH-HA and APOBEC1-YTHmut-HA proteins were in vitro transcribed/translated using the Promega TNT T7 Quick Coupled In Vitro Transcription/Translation kit. Briefly, 1 μg of plasmid DNA was used in a 50 μl reaction and incubated for one hour at 30° C. 5 μl of each reaction was then mixed with 30 ng of a 1500 nt-long RNA with a single internal A or m⁶A site, 0.5 μl RNasin (Promega (Madison, Wis.)), in 1X deaminase buffer (10 mM Tris-HCl pH7.5, 50 mM KCl, 0.1uM ZnCl₂). Reactions were incubated for 4 h at 37° C. RNA was isolated with the Qiagen RNeasy Plus Mini kit and treated with DNase I (New England Biolabs) for 15 min at 37° C. For assays using cellular RNA, in vitro deamination was carried out for 6 h at 37° C. using 50 ng of total RNA from HEK293T cells. Sequencing libraries were then prepared using the NebNext Ultra II Directional RNA Library Prep Kit for Illumina (New England Biolabs (Ipswich, Mass.)).

cDNA synthesis and Sanger sequencing. 1 μg purified RNA from in vitro deamination assays or from cells expressing APOBEC1-YTH or APOBEC1 alone was used for cDNA synthesis using either a 1:1 mix of oligo(dT) and random hexamers or gene specific primers (below). cDNA synthesis was carried out using the SuperScript III reverse transcriptase kit according to manufacturer's instructions (Thermo). PCR was then performed using Phusion High-Fidelity PCR Mastermix (New England Biolabs) and primers flanking m⁶A target regions. Purified PCR products were then either directly sequenced using Sanger sequencing or cloned into the pCR Blunt II TOPO vector (Thermo). For direct Sanger sequencing, measurements of C to U conversion were quantified from Sanger sequencing traces by calculating the height of T sequence peaks relative to C sequence peaks at individual mixed (C/T) sites. For TOPO cloning, a minimum of three individual clones were selected for each condition, and a representative Sanger sequencing trace for each is shown. Sequencing primers were used as indicated and comprised the sequences listed below.

Primers (5′-3′). The primers used in these studies are listed in the table below.

YTH Fwd AGACTCCCGGGACCTCAGAG (SEQ ID NO: 36) YTH^(mut) Fwd ACTCCCGGGACCTCAGAGTCCGCCACACCAGAAGGCCGGGTTTTC ATCATTAAG (SEQ ID NO: 37) YTH-HA Rev CGGGTTTAAACTCAGGCGTAGTC (SEQ ID NO: 38) BSG RT primer GTGGGGGCGATCTTTATTGTGGCGG (SEQ ID NO: 39) ACTB RT primer TGTGCAATCAAAGTCCTCGGCCAC (SEQ ID NO: 40) BSG Fwd/Sanger GCCAATGCTGTCTGGTTGCGCC (SEQ ID NO: 41) BSG Rev GGAGGCTTCTGCGGTTCTGGAG (SEQ ID NO: 42) ACTB Fwd/Sanger CAGCAAGCAGGAGTATGACGAGTC (SEQ ID NO: 43) ACTB Rev CATGCCAATCTCATCTTG (SEQ ID NO: 44) ACTB MeRIP Fwd CATGTACGTTGCTATCCAGGC (SEQ ID NO: 45) ACTB MeRIP Rev CTCCTTAATGTCACGCACGAT (SEQ ID NO: 46) ATRX MeRIP Fwd CGAAGATCCCCACGTGTAAAGACTAC (SEQ ID NO: 47) ATRX MeRIP Rev CATCCTGCTCACCTCTTTGAGG (SEQ ID NO: 48) BPTF MeRIP Fwd GTGTTAGATGATGTCTCCATTCGGAG (SEQ ID NO: 49) BPTF MeRIP Rev CACTTTCCTCCTGTATGAGCGG (SEQ ID NO: 50) Single A 1500 nt  GCCAAGAGGCAACACACCAAC (SEQ ID NO: 51) RNA RT primer Single A 1500 nt  CGGTTTCTCTCGGTCTGTTTTCC (SEQ ID NO: 52) RNA Fwd Single A 1500 nt  CAGAAGGCGACAACACAGCAACACC (SEQ ID NO: 53 RNA Rev

Next-generation sequencing. All sequencing was performed by the Duke University Sequencing and Genomic Technologies Core facility. HEK293T cells were transfected with APOBEC1-YTH, APOBEC1-YTH^(mut), or APOBEC1 alone using FuGENE HD according to the manufacturer's instructions (Promega). After 24 h, total RNA was isolated with TRIzol (Thermo) and subjected to DNase I treatment using RNase-free DNasel (Sigma) for 20 min at 37° C. 1 μg of total RNA was then used for sequencing library preparation using the NebNext Ultra II Directional RNA Library Prep Kit for Illumina (New England Biolabs). For low-input samples, the Single Cell/Low Input RNA Library Prep Kit (New England Biolabs) was used with either 10 ng or 100 ng of total RNA as input as indicated. It was not necessary to remove rRNA prior to sequencing library preparation, although doing so may potentially be used to further increase the efficiency of C to U mutation detection. Prior to sequencing, samples were barcoded using NEBNext Multiplex Oligos for Illumina (New England Biolabs). Libraries were then sequenced on the Illumina Hi Seq 4000. For PacBio sequencing, 1 μg of RNA was used for library preparation using the Iso-Seq system. Two samples were sequenced on one SMRT cell of the PacBio Sequel instrument (Menlo Park, Calif.) and processed using the Iso-Seq analysis pipeline.

C to U editing site analysis. Sequencing reads were demultiplexed, adapters were removed, and strand-specific reads were reverse-complemented and aligned to the human genome (hg19) using Novoalign. PCR duplicates were collapsed, and individual C to U mutations were identified using CIMS (Shah et al. CLIP Tool Kit (CTK): a flexible and robust pipeline to analyze CLIP sequencing data. Bioinformatics 33, 566-567 (2017)) C to U sites identified with the p<1 threshold were further filtered, and only those sites that had a minimum of 2 mutations, at least 10 reads per replicate, and a mutation/read (m/k) threshold of 10-60% (for high-stringency lists) were kept. It was found that adjusting the number of mutations, reads per replicate, and m/k threshold is a good way to increase/decrease stringency of m⁶A site calls to a desired level. If desired, sites can be further filtered to include only C to U editing events which are immediately preceded by an A; however, this could potentially exclude some m⁶A sites for which editing occurs at a nearby C instead of the immediately adjacent C. In addition to these filtering steps, known mutations in the human genome (dbSNP 150), as well as endogenous C to U editing sites identified by sequencing of wild type HEK293T cells, were also removed. For APOBEC1-YTH or APOBEC1-YTH^(mut) expressing cells, the list of C to U editing sites was further processed by removing sites detected in cells expressing APOBEC1 alone. For determining enrichment of C to U editing between samples, a filter of m/k was used to find sites that were of the indicated fold-enrichment greater than the reference sample. PacBio datasets were aligned to the human genome (hg19) using GSNAP (Wu et al. GMAP: a genomic mapping and alignment program for mRNA and EST sequences. Bioinformatics 21, 1859-1875 (2005)).and subjected to the same pipeline as above to identify C to U sites. In vitro DART-Seq datasets were also subjected to the same C to U mutation analysis using a m/k filtering threshold of 5-60%.

Exon length measurements. Exon length was determined using the RefSeq hg19 annotation. Sequencing reads spanning individual exons were processed by removing first and last exons, according to the consensus RefSeq annotation. In cases where reads overlapped with multiple isoforms and therefore different exons, the consensus Refseq sequence was used.

Metagene and motif analyses. Metagene analysis was performed using hg19 annotations according to previously published methods (Olarerin-George et al. MetaPlotR: a Perl/R pipeline for plotting metagenes of nucleotide modifications and other transcriptomic sites. Bioinformatics 33, 1563-1564 (2017)). Discovery of enriched motifs was performed using HOMER (Heinz et al. Simple combinations of lineage-determining transcription factors prime cis-regulatory elements required for macrophage and B cell identities. Mol Cell 38, 576-589 (2010)) using sequences spanning a region 4 nucleotides up- and downstream of C to U editing sites as input.

Replicates analysis. Independent biological replicates of global DART-Seq experiments were compared by computing the Pearson correlation coefficient between the number of C to U mutations per gene between any two replicate experiments.

Calculating C to U editing events in individual reads. To determine the number of reads with one or more C to U editing events out of all reads that spanned at least two called C to U editing events, Sam2Tsv (Lindenbaum jVarkit:j ava-based utilities for Bioinformatics (2015)) for was used to identify individual reads containing C to U mutations. The first and last position of each read was then used in conjunction with bedtools intersect (Quinlan and Hall, I.M. BEDTools: a flexible suite of utilities for comparing genomic features. Bioinformatics 26, 841-842 (2010)) to find reads that overlap more than one C to U editing site from our final list of high-confidence sites. The number of editing events within these reads was then counted and summed.

Dataset comparisons. DART-Seq and m⁶A immunoprecipitation (MeRIP-Seq, miCLIP) datasets were analyzed using the closest and intersect features of the bedtools suite. For comparison of methylated mRNAs, bed file coordinates were annotated using the annotation feature of the metagene analysis pipeline (above) to give individual mRNAs, which were then compared between datasets.

Statistics. Statistical analysis of cell viability and western blot data were performed using a two-tailed t-test. Analysis of C to U editing enrichment in various transcript regions following CPT treatment, as well as analysis of the proportion of C to U sites following each nucleotide, was performed using a chi-squared test.

B. Results

-   -   1. APOBEC1-YTH Converts m⁶A Adjacent Cytidine to Uridine

The APOBEC1-YTH fusion protein was incubated with a synthetic RNA containing a single internal adenosine. Reverse transcription and Sanger sequencing indicated frequent editing of the cytidine immediately following m⁶A in methylated RNA, but not in unmethylated RNA. APOBEC1-YTH^(mut), which lacks the m⁶A-binding portion of the YTH domain, failed to convert C to U in m⁶A-containing RNA.

To confirm that the observed editing was caused by targeting of APOBEC1 to the m⁶A residue, the in vitro deamination assays was repeated using a mutant version of the APOBEC1-YTH fusion protein (APOBEC1-YTH^(mut)) in which the m⁶A binding region of the YTH domain was deleted (FIG. 2). APOBEC1-YTH^(mut) was impaired in its ability to bind m⁶A (FIG. 3) and failed to convert adjacent cytidines to uridines in m⁶A-containing RNA indicating that the deaminase activity of APOBEC1-YTH is directed by the m⁶A-binding activity of the YTH domain.

-   -   2. DART-Seq Enables Transcriptorne-Wide Detection of m6A

To determine whether APOBEC1-YTH could be used to detect endogenous m⁶A sites in cells, DART-Seq (deamination adjacent to RNA modification targets) was developed, in which APOBEC1-YTH was introduced into cells and then total RNA was subjected to next-generation sequencing followed by C to U mutation detection (FIG. 1). Comparison of three biological replicates indicated high reproducibility in C to U mutations in APOBEC1-YTH-expressing HEK293T cells, suggesting that APOBEC1-YTH targets specific RNAs for editing with high consistency across samples.

To determine whether DART-Seq can identify m⁶A residues, C to U editing sites from cells expressing APOBEC1-YTH were compared to cells expressing APOBEC1 alone. DART-Seq editing events from APOBEC1-YTH-expressing cells occurred primarily in the 3′UTR and coding sequence (CDS) and were enriched in the vicinity of the stop codon which mirrors the distribution of m⁶A. In contrast, editing events from cells expressing APOBEC1 alone were located primarily in 3′UTRs and intergenic regions and failed to show an enrichment near the stop codon. Furthermore, there was little overlap in C to U editing between the two datasets, as 96% of edited sites from APOBEC1-YTH-expressing cells were not detected in cells expressing APOBEC1 alone (56,603 out of 59,246 sites)

To further ensure that C to U editing was caused by recruitment of APOBEC1-YTH to m⁶A, RNA-seq on HEK293T cells expressing APOBEC1-YTH^(mut) was performed and the same C to U editing analysis carried out. C to U editing events in cells expressing APOBEC1-YTH^(mut) showed a distinct distribution compared to those in cells expressing APOBEC1-YTH, characterized by an enrichment throughout the 3′UTR as opposed to in the vicinity of the stop codon (FIG. 4) Together, these results suggest that the specificity of APOBEC1-YTH editing throughout the transcriptome depends on its ability to bind m⁶A.

To obtain a set of high-confidence editing sites, the list of APOBEC1-YTH sites was filtered to include only those with at least a 1.5-fold enrichment over APOBEC1-YTH^(mut) samples. All naturally occurring C to U mutations in HEK293T cells were also excluded, as well as C to U editing sites detected in cells expressing APOBEC1 alone. This resulted in a list of 100,636 C to U editing sites in 9,793 RNAs that occurred in at least 5% of all reads. Of these, a stringent list of 40,263 editing events in 7,707 RNAs was observed in at least 10% of all reads.

Examination of sequences immediately surrounding DART-Seq sites revealed enrichment of a GGACU-containing motif, which matches the preferred consensus sequence for m⁶A. In contrast, motifs detected in APOBEC1-YTH^(mut) and APOBEC1 samples did not match the m⁶A consensus. Furthermore, DART-Seq sites were highly enriched within 3′UTRs and in the vicinity of the stop codon, as well as within long internal exons (FIG. 5), which matches the distribution of m⁶A. Comparison of methylated RNAs detected by MeRIP-Seq and those identified by DART-Seq showed a high degree of overlap, with 64% of m⁶A-containing RNAs detected by DART-Seq (3,679 of 5,768 RNAs). Examination of individual RNAs showed that DART-Seq editing events occurred at sites of MeRIP-Seq enrichment. Furthermore, consistent with the in vitro deamination assays, C to U editing events frequently occurred immediately downstream of known m⁶A sites in cellular RNAs.

-   -   3. Comparison Between DAR1:-Seq and Antibody-Based Approaches

Next, the ability of DART-Seq to identify individual m⁶A sites compared to antibody-based approaches was assessed. Comparison of global m⁶A profiling datasets obtained by m⁶A immunoprecipitation showed that DART-Seq performs similarly in its ability to detect m⁶A sites (FIGS. 6a and 6b ). It also showed an enrichment of DART-Seq editing adjacent to m⁶A sites identified by single-nucleotide resolution m⁶A profiling (miCLIP/m⁶A-Seq). Additionally, the majority (91.4%) of C to U editing sites in APOBEC1-YTH-expressing cells are preceded by an A, compared to only 67.9% of C to U editing sites in cells expressing APOBEC1 alone, suggesting that APOBEC1-YTH deamination is directed specifically toward cytidines adjacent to m⁶A and that promiscuous editing of non-adjacent cytidines is rare. Further support for this comes from the finding that over 90% of DART-Seq sites are greater than 10 nucleotides away from the closest editing event, which is similar to the distribution seen in miCLIP (FIG. 6c ). Collectively, these data indicate that DART-Seq is capable of detecting m⁶A sites in cellular RNAs transcriptome-wide.

-   -   4. Validation of the DART-Sea Approach

To validate individual DART-Seq sites, RT-PCR and Sanger sequencing was performed to determine whether C to U editing occurs adjacent to m⁶A sites previously quantified by miCLIP⁵ or by SCARLET¹⁶ , another single-nucleotide resolution m⁶A identification method. The presence of editing adjacent to known m⁶A sites in the BSG and ACTB mRNAs in cells expressing APOBEC1-YTH was confirmed, but no robust editing in cells expressing APOBEC1 alone was observed. To further validate that DART-Seq editing depends on the presence of m⁶A, DART-Seq using HEK293T cells depleted of the m⁶A methyltransferase, METTL3 was performed. METTL3-depleted cells exhibited fewer DART-Seq editing events in general and loss of the GGACU m⁶A consensus sequence surrounding DART-Seq sites. Furthermore, 97% of the DART-Seq sites detected in wild type cells were lost in METTL3-depleted cells (FIGS. 7a and 7b ) These results further confirm that DART-Seq editing depends on m⁶A.

-   -   5. DART-Seq Enables Low-Input Global m6A Profiling

One of the biggest challenges for global m⁶A detection has been the large amount of input RNA required for effective immunoprecipitation and sequencing. Recent advances in library preparation have provided important improvements, with some studies reporting m⁶A profiling using as little as 150 ng of mRNA or 500 ng of total RNA. However, even with such improvements, the requirement for high nanogram amounts of poly(A) or rRNA-depleted RNA can be limiting for certain cell or tissue types.

It was therefore tested whether DART-Seq could be used to detect m⁶A in low-input RNA samples. Using as little as 10 nanograms of total RNA as input, over 79% of the DART-Seq edited mRNAs that were identified in the high-input DART-Seq library were detected. Low-input DART-Seq samples perform similarly to antibody-based approaches for m⁶A detection, albeit with slightly reduced efficiency compared to high-input DART-Seq samples (FIG. 8). In addition, low-input DART-Seq sites are enriched for m⁶A consensus motifs and near the 5′ end of the 3′UTR. Thus, DART-Seq is capable of detecting m⁶A sites from as little as 10 nanograms of total RNA.

-   -   6. m6A Detection Using in vitro DART-Seq

APOBEC1-YTH-expressing cells exhibit normal levels of genes in the m⁶A regulatory pathway and show no alterations in cell viability, suggesting that prolonged APOBEC1-YTH expression does not alter the m⁶A landscape (FIG. 9a-c ). Nevertheless, APOBEC1-YTH overexpression may not be possible or desirable in some cases, which would necessitate the use of in vitro deamination to perform DART-Seq. To test the ability of this approach to detect m⁶A in cellular RNA, in vitro DART-Seq using HEK293T cell RNA was performed. C to U editing at known m⁶A sites was detected, and global analyses revealed a distribution and motif enrichment similar to that of m⁶A (FIG. 10). Although the majority (91%) of methylated mRNAs identified with in vitro DART-Seq were also identified using cellular DART-Seq, in vitro DART-Seq identified fewer methylated mRNAs than cellular DART-Seq, suggesting reduced efficiency. Thus, in vitro DART-Seq can reliably mark m⁶A sites in cellular RNAs, although this approach will likely benefit from further optimization to increase identification of low-abundance m⁶A sites.

-   -   7. DART-Seq Distinguishes m6A From m6Am

A limitation of antibody-based m⁶A detection strategies is cross-reactivity of m⁶A antibodies with m⁶Am. Hydrogen bonding between the YTH domain and the 2′-OH of m⁶A suggests that the YTH domain used in DART-Seq may not recognize m⁶Am. Furthermore, unlike m⁶A residues, m⁶Am is not invariably following by a cytidine, which means that detection of m⁶Am by APOBEC1-YTH would require deamination of cytidines further away from the modified base. It was, therefore, tested whether DART-Seq could be used to distinguish m⁶A from m⁶Am.

To investigate this, a list of m⁶Am sites in HEK293 cells was compared to DART-Seq datasets. Since m⁶A sites in 5′UTRs may actually reflect m⁶Am residues at misannotated start sites, the DART-Seq sites were extended to include regions 4 nt up- and downstream from the C to U editing site (Linder, B. et al. Single-nucleotide-resolution mapping of m6A and m6Am throughout the transcriptome. Nat Methods 12(8): 767-72 (2015)). Only one RNA with overlap between extended DART-Seq sites and m⁶Am sites was found. Upon closer examination, the DART-Seq editing site in this transcript is diminished in METTL3-depleted cells and is found internally within the 5′UTR, suggesting that it is not an m⁶Am site. This suggests that DART-Seq does not recognize m⁶Am and can be used to identify m⁶A residues independently of m⁶Am residues.

-   -   8. Estimation of m6A Abundance.

Determining m⁶A abundance within individual RNAs has been a major challenge to RNA methylation research. SCARLET enables quantitative measures of m⁶A in individual RNAs, but this approach is not amenable to transcriptome-wide measurements. m⁶A-LAIC-Seq uses immunoprecipitation of full-length transcripts to estimate methylation levels of individual mRNAs, but it does not account for multiple m⁶A sites or the presence of m⁶Am. Finally, peak over input (POI) can be used in MeRIP-Seq, but these measures provide only a rough estimate of m⁶A abundance.

It was speculated that the degree of methylation may correlate with APOBEC1-YTH binding and C to U editing to enable global estimates of m⁶A abundance in individual transcripts. To test this, in vitro deamination assays were performed using RNA with various amounts of m⁶A. C to U editing was positively correlated with m⁶A levels at individual sites within an RNA (FIG. 11). Examination of DART-Seq editing of cellular RNAs also showed a positive relationship between m⁶A abundance and editing efficiency. Thus, DART-Seq can be used as an indicator of m⁶A abundance in individual RNAs.

-   -   9. DART-Seq Identifies m6A Accumulation in Cellular RNAs

Next, it was tested whether changes in m⁶A can be detected by DART-Seq. Previous studies have shown that treatment of cells with moderate concentrations of the topoisomerase inhibitor camptothecin (CPT) causes slowed transcription and an increase in m⁶A abundance in the CDS. HEK293T cells expressing APOBEC1-YTH were treated with CPT for 5 h and DART-Seq was performed to identify m⁶A sites. This led to 6,258 C to U sites that showed at least a 2-fold increase in editing compared to untreated cells. Metagene analysis of these sites indicated a slight enrichment in the CDS compared to untreated cells (FIG. 12a ), and examination of individual mRNAs confirmed this analysis. The increase in m⁶A within the CDS of select mRNAs was validated using m⁶A immunoprecipitation followed by RT-qPCR (MeRIP-RTqPCR) (FIG. 12b ). Increased m⁶A in these RNAs also negatively correlated with their abundance (FIG. 12c ), similar to what has been previously observed. Thus, DART-Seq can be used to detect accumulation of m⁶A in individual RNAs in response to changing cellular conditions.

-   -   10. Long-Read DART-Seq Reveals Isoform-Specific Methylation         Patterns

Immunoprecipitation-based m⁶A detection strategies have previously reported clustering of m⁶A sites. However, it remains unknown whether this reflects clustering of m⁶A on the same or distinct RNA molecules. Since DART-Seq induces editing events in single transcripts, it was reasoned that individual sequencing reads could be examined to determine whether m⁶A sites are found in the same RNA molecule. To investigate this, long-read DART-Seq was performed using the PacBio platform. Examination of individual mRNAs showed that, although some transcripts exhibit isoform-specific regional editing, others contain DART-Seq sites in the 5′UTR, CDS, and 3′UTR. In addition, 41% of reads spanning at least two editing sites contain two or more C to U editing events. These data suggest that the majority of individual RNA molecules have just one m⁶A site, but that many RNAs harbor multiple sites, which is consistent with previous reports from isoform-specific m⁶A immunoprecipitation (m⁶A-LAIC-seq). Further studies will be needed to understand whether distinct m⁶A residues on the same transcript work in a coordinated or competing manner. Additionally, although our data suggest that multiple C to U editing events caused by the same m⁶A site are rare (FIG. 6c ), studies of clustered m⁶A sites in individual transcripts may benefit from additional validation using miCLIP or SCARLET.

Example 2. Detecting m⁶A in Single Cells and Brain Tissue

A. Materials and Methods

Vector Construction. The pET-His6-MBP-TEV-LIC plasmid (Addgene 29656) was linearized by digestion with SspI and gel purified. The APOBEC1-YTH and APOBEC1-YTHmut sequences were PCR amplified with the addition of a 5′ sequence (TACTTCCAATCCAATGCA)(SEQ ID NO: 54) and a 3′ sequence (TTATCCACTTCCAATGTTATTA) (SEQ ID NO: 55). 300 ng of the linearized vector was then treated with 0.6 μL of T4 Polymerase (NEB) in a buffer containing 1X NEB Buffer 2.1 and 2.5 mM dGTP. The solution was incubated at 22° C. for 20 minutes, and 75° C. for 15 minutes. 0.2 pmol of the insert was then treated with 0.4 μL of T4 Polymerase (NEB) in a buffer containing 1X NEB Buffer 2.1 and 2.5 mM dCTP. The solution was incubated at 22° C. for 20 minutes, and 75° C. for 15 minutes. The insert was then annealed to the linearized vector by incubating 3 μL of the vector reaction with 2 μL of the insert reaction for 5 minutes at room temperature. Then 1 μL of 25 mM EDTA was added and 3 μL was used to transform DH5α cells. Successful cloning was validated by restriction digestion and Sanger sequencing of the insert sequence. The plasmids were then purified from DH5a cells and transformed into Rosetta 2 (DE3) pLYS cells. The resulting plasmids express a 6×His-MBP-APOBEC1-YTH-HA fusion protein under control of the lac operator promoter so that it is inducible with lactose or IPTG.

Bacterial Induction and lysis. Rosetta 2 pLYS cells containing plasmids coding for 6×His-MBP-APOBEC1-YTH-HA or 6×His-MBP-APOBEC1-YTHmut-HA were cultured overnight in 10 mL of LB broth. The next morning, 5 mL of overnight culture was added to 1 L of ZY induction medium (1% tryptone, 0.5% yeast extract, 25 mM Na₂HPO₄, 25 mM KH₂PO₄, 50 mM NH₄Cl, 5 mM Na₂SO₄, 0.5% glycerol, 0.05% glucose, 0.2% α-lactose, 2 mM MgSO₄) containing 100 μg/mL kanamycin and cultured for approximately 10 hours at 37° C. with shaking in baffled flasks. Cultures were then grown at 20° C. with shaking for 16 hours. Expression was induced as the culture consumes the glucose and begins to import the lactose. After 16 hours, the cells were pelleted by centrifugation at 5,000g for 15 minutes at 4° C. The pellets were then flash frozen in liquid nitrogen and thawed at 4° C. Once thawed the cells were lysed using the Qproteome Bacterial Protein Prep Kit (Qiagen). Briefly, the pellet from 1 L of culture was resuspended in 40 mL of lysis buffer (40 mL Native Lysis Buffer, 40 mg lysozyme, 1 U benzonase) by pipetting while on ice. After resuspension, the lysate was incubated on ice for 30 minutes, with gentle swirling every 10 minutes. Then the lysate was cleared of insoluble material by centrifugation at 10,000 g for 60 minutes at 4° C. and the supernatant was moved to a fresh tube and stored on ice.

Protein purification. 750 μL of resuspended Ni-NTA resin (Gold Bio) was pipetted into a 10 mL polypropylene chromatography column (Bio-Rad) and the liquid drained by gravity flow. Once drained, 6 mL of equilibration buffer (50 mM Na₂HPO₄, 100 mM NaCl, 10 mM Imidazole) was added and allowed to drain over the resin by gravity flow. Once drained, the lysate was added to the column and allow to pass over the resin by gravity flow. After the lysate completely passed through the resin, 10 mL of wash buffer 1 (PBS with 10 mM imidazole, 5 mM (3-mercaptoethanol (BME)) was added and allow to drain by gravity flow. The resin was then rinsed with two 6 mL washes of wash buffer 2 (PBS with 25 mM imidazole, 5 mM BME), two 6 mL washes of wash buffer 3 (PBS with 50 mL imidazole, 5 mM BME), and two 6 mL washes of wash buffer 4 (PBS with 60 mM imidazole, 5 mM BME). The protein was then eluted from the resin with 1 mL of elution buffer 1 (10 mM Tris-HCl (pH 7.4), 100 mM NaCl, 250 mM imidazole, 5 mM BME), followed by 1 mL of elution buffer 2 (10 mM Tris-HCl (pH 7.4), 100 mM NaCl, 300 mM imidazole, 5 mM BME), and 1 mL of elution buffer 3 (10 mM Tris-HCl (pH 7.4), 100 mM NaCl, 500 mM imidazole, 5 mM BME). Elution 1 and 2 were determined to contain a vast majority of the protein, so elution 1 and 2 were pooled and dialyzed in a 0.5-3 mL Slide-a-Lyzer dialysis cassette with a 3.5 KD MWCO (Thermo-Fisher). The 2 mL eluate was added to the cassette, which was placed in 1 L of storage buffer (10 mM Tris-HCl (pH 7.4), 100 mM NaCl, 1 mM DTT) at 4° C. for 2 hours. After 2 hours, the cassette was placed in 1 L of fresh storage buffer and dialyzed for 16 hours. After dialysis, the sample was removed from the cassette and diluted by adding an equal volume of glycerol. Aliquots were stored at −20° C. Protein levels of APOBEC1-YTH and APOBEC1-YTH^(mut) are shown using Coomassie staining (FIG. 13).

In vitro DART with purified protein In vitro DART-seq was performed on total RNA isolated from cultured HEK293T cells, primary mouse hippocampal neurons, whole brain lysates, and synaptoneurosome preparations taken from the mouse striatum. DART assays were carried out by combining 250 ng purified APOBEC1-YTH or APOBEC1-YTHmut protein with 30-50 ng total RNA in a 50 ul reaction with 5 ul 10X DART Buffer (500 mM KCl, 100 mM Tris-HCl, pH 7.4, 1 uM ZnCl₂) and 1 ul RNaseOUT. Samples were gently mixed with a pipette and incubated at 37° C. for 4 h. RNA was then isolated with the RNeasy Micro Kit (Qiagen). For Sanger sequencing, cDNA was generated using SuperScript III (Thermo Fisher) or iScript (Bio-Rad) according to manufacturer's instructions. For next-generation sequencing, libraries were prepared as indicated.

Transient transfection and cell sorting. To obtain single-cell preparations of APOBEC1-YTH or APOBEC1-YTHmut expressing cells, HEK293T cells were transiently transfected with pCMV-APOBEC1-YTH-IRES-EGFP or pCMV-APOBEC1-YTHmut-IRES-EGFP using Lipofectamine 2000 according to the manufacturer's instructions (Thermo Fisher). After 24 hours, cells were removed from the plate with TrypLE (Thermo Fisher) and resuspended in DMEM/1% FBS at a dilution of 200,000 cells/mL. Cells were then treated with 10 U DNasel (NEB) for 15 min at 25° C. and washed with 2 mL DMEM/1% FBS with 1 U Dnasel. 5 uL of lysis buffer (NEBNext Single cell/Low Input Library Prep kit) was added to each well of a 96-well plate. Cells were then sorted using a A02 FACSAriaII, with 1 cell being sorted in each of 8 wells of a 96 well PCR plate. This was done for APOBEC1-YTH and APOBEC1-YTHmut expressing cells. Immediately after sorting, plates were flash frozen on dry ice and stored at −80° C. until further processing. Sequencing library preparation was performed using the NEBNext Single Cell/Low Input Library Prep Kit according to the manufacturer's instructions (NEB).

Single-cell DART-seq (scDART-seq). Sequencing library preparation using the NEB Single-cell/Low Input Library Prep Kit for Illumina (library preparation) and NEBNext Index Primer Sets (Set 1 and Set 4 were used) was performed according to manufacturer's protocol. The pipetting steps for reverse transcription and cDNA amplification were performed in a biosafety cabinet. Briefly, 1 μL of RT primer mix and 3 μL of nuclease-free water was added to each well with lysis buffer and incubated at 70° C. for 5 minutes. Then, 11 μL of reverse transcription mix (5 μL RT buffer, 1μL template switching oligo, 2 μL RT enzyme mix, and 3 μL nuclease-free water) was added to each well and incubated at 42° C. for 90 minutes, then 70° C. for 10 minutes. cDNA was amplified by adding 50 μL cDNA PCR master mix, 2 μL cDNA PCR Primer, and 28 μL of nuclease-free water to each well and incubated in a thermocycler with following conditions (98° C. for 45 seconds, then 18 cycles of: 98° C. for 10 seconds, 62° C. for 15 seconds, 72° C. for 3 minutes, then 72° C. for 5 minutes). The cDNA was cleaned using SPRI Select beads (0.6X of sample volume) and a magnetic tube holder. The beads were washed twice with 80% ethanol. The cDNA was then eluted with 504, 0.1X TE before being re-bound to the beads using the provided Bead Reconsitution Buffer and washed twice again with 80% ethanol. The cDNA was eluted into 30 μL of 0.1X TE buffer. cDNA quality and quantity was determined using a High Sensitivity DNA Bioanalyzer Assay. Successful cDNA preps were fragmented by adding 26 μL of purified cDNA, 7 μL FS Reaction Buffer, and 2 μL FS Enzyme Mix and incubating in a thermocycler at 37° C. for 25 minutes and 65° C. for 30 minutes. Adaptor ligation was performed by adding 30 μL Ligation Master Mix, 1 μL Ligation Enhancer, and 2.5 μL of NEBNext Adaptor for Illumina (1:25 diluted) and incubating at 20° C. for 15 minutes. Then 3 μL of USER Enzyme was added and incubated at 37° C. for 15 minutes. The DNA was cleaned using SPRI Select beads (0.8X sample volume) and washed 2X with 80% ethanol and eluted with 15 μL of 0.1X TE. Libraries were amplified and indexes for multiplexing added by adding 15 μL DNA fragments, 25 μL Q5 Master Mix, 5 μL Universal Primer, and 5 μL i7 Primer and incubating in a thermocycler with the following conditions (98° C. for 30 seconds, then 8 cycles of: 98° C. for 10 seconds, 65° C. for 75 seconds, then 65° C. for 5 minutes. The indexed libraries were cleaned using SPRI Select beads (0.9X sample volume) and washed 2X with 80% ethanol and eluted with 15 μL of 0.1X TE. Library quality and quantity was determined using a High Sensitivity DNA Bioanalyzer Assay. Libraries were pooled and sequenced at a concentration of 10 nM on a Novaseq6000. Quality filtering, alignment, and C to U analysis was performed as described (Meyer, Nature Methods 2019). Metagene analysis and consensus sequence analysis was performed as described (Meyer, Nature Methods 2019) and revealed an enrichment of C to U editing in the vicinity of the stop codon and near GAC or AAC sequences as has been observed for m⁶A.

Stable cell lines. Stable HEK293T cell lines were generated by lentiviral infection and antibiotic resistance selection. APOBEC1-YTH and APOBEC1-YTHmut were cloned into the TLCV2 lentiviral vector (Addgene 87360) replacing the Cas9 sequence using Gibson Assembly. The resulting lentiviral vectors contained the TRE-Tight promoter (containing a Tet-response element just upstream from a minimal CMV promoter) driving expression of APOBEC1-YTH-T2A-EGFP or APOBEC1-YTHmut-T2A-EGFP.

Lentiviral production. 4 μg of lentiviral vector DNA was combined with 2 μg psPAX packaging vector and 1 μg pMD2.G coat protein plasmid in 1 mL of OPTI-MEM. 21 μL of PEI was added and the mixture was incubated at room temperature for 25 minutes. It was then added dropwise to a 10 cm plate of HEK293T cells at 80% confluency cultured in full DMEM with 10% FBS. After 24 hours the medium was replaced with 10 mL of fresh DMEM with 10% FBS. 72 hours post transfection, the media was collected and centrifuged at 600 g for 10 minutes before the supernatant was passed through a 0.45 μm cellulose filter to remove large debris. Approximately 9 mL of medium was added to ultracentrifuge tubes (Beckman Coulter Fef: 331374). Then 1.5 mL of chilled, sterile 20% glucose was carefully added below the medium to create two separate phases. Purified virus was pelleted by centrifugation at 19,7000rpm for 2 hours at 4° C. in a Beckman SW40 swinging bucket rotor. After centrifugation, the supernatant was removed and the tube was allowed to air dry for 10 minutes. When dry, 100 μL of PBS was added and the virus was resuspended at 4° C. overnight, with rocking. The next day, 10 μL aliquots were made and stored at −80° C. until use.

Viral titering. 3,000 HEK293T cells were plated in each well of a 96-well plate and cultured for 24 hours in DMEM with 10% FBS. Then purified virus was added at increasing concentrations (1:5, 1:25, 1:125, 1:625, 1:3125). 24 hours after adding the virus, the medium was replaced with fresh DMEM with 10% FBS containing 1 μg/mL doxycycline to induce expression. 6 hours after doxycycline treatment, GFP signal intensity was observed by fluorescence microscopy using a Leica DMi8 inverted microscope.

Infection and selection of stable cells. HEK293T cells were plated onto 6 well plates and cultured in DMEM with 10% FBS. Viral stock (1:500 for APOBEC1-YTHmut, 1:100 for APOBEC1-YTH) was then added. After 24 hours, medium was changed, and another 24 hours after that, medium was replaced with selective medium (DMEM/10% FBS with 5 ug/mL puromycin). Fresh medium was added every 48 hours. After 7 days of puromycin selection, cells were diluted and plated with an average density of 1 cell per well across several 96 well plates. Individual wells were screened to identify wells with just a single cell. Colonies were then expanded from these clonal isolates. Expression of APOBEC1-YTH or APOBEC1-YTHmut was induced by treating cells with 1 μg/mL doxycycline and confirmed by EGFP expression and anti-HA western blot. C to U deamination at known m⁶A sites in the ACTB and EEF2 mRNAs was tested with RT-PCR and Sanger sequencing and could be detected as early as 4 hours after dox addition. Complete editing was observed after 16-24 hours of dox treatment.

B. Results

-   -   1. Alternative deaminase enzymes improve in6A detection.

To test the effect of deaminase variants on m6A detection the cytidine deaminases APOBEC1, AID, and APOBEC3A were all fused to the YTH domain of YTHDF2 and expressed in HEK293T cells for 24 h. Total RNA was isolated and subjected to RT-PCR and Sanger sequencing targeting methylated regions of the ACTB and EEF2 mRNAs. FIG. 14 shows the % C to T conversion (in cDNA sequences) for each fusion protein. The AID-YTH variant causes higher editing rates in the EEF2 mRNA compared to APOBEC1 and APOBEC3A.

-   -   2. DART-seq Detects Alternative Methylation of Synaptic mRNAs         From Mouse Brain Tissue

In vitro DART-Seq Was Performed on RNA Isolated From Whole Brain or Synaptic fractions (SN). FIG. 15 shows Sanger sequencing traces at two m⁶A sites in the 3′UTR of Arc, an mRNA enriched near synapses. Arrows indicate the cytidine adjacent to the m⁶A site. DART-seq induced C to U mutations (C to T in cDNA) show that site A1682 is selectively methylated in whole brain samples, whereas site A1469 is selectively methylated in the SN fractions, suggesting that DART-seq can detect localization-specific methylation.

-   -   3. In vitro DART-seq Detects m6A Sites That are Identified by         Cellular DART-Seq

In vitro DART-seq was performed using HEK293T cell total RNA and compared to DART-seq data obtained from HEK293T cells expressing APOBEC1-YTH for 24 h. RT-PCR and Sanger sequencing were performed to show C to U mutations (shown as C to T in the cDNA sequence) adjacent to two m⁶A sites in the ACTB mRNA and one m⁶A site in the EEF2 mRNA. Table 1 shows the mutations rates (%T) for cells expressing APOBEC1-YTH (labeled as Transfected) and for total cellular RNA subjected to in vitro DART-seq (labeled as In vitro). The results indicate that In vitro DART-seq mutation rates are comparable to cellular DART-seq mutation rates.

TABLE 1 Transfected and in vitro DART-seq mutation rates ACTBA1222 ACTBA1248 EEF2A2982 Transfected (% T) 16.4 39.8 13.2 In vitro (% T) 16.1 35.3 12.3

4. Single Cell DART-seq (scDART-seq) Detects m6A in Single Cells

HEK293T cells were transfected with APOBEC1-YTH or APOBEC1-YTHmut plasmids co-expressing EGFP. FACS sorting was used to isolate single cells, and RNA-seq libraries were generated and libraries sequenced with Illumina short read sequencing. Table 2 (see below) shows editing (as % C to T reads) at cytidine residues adjacent to m⁶A sites in the ACTB and SPEN mRNAs. Both mRNAs show high C-to-T mutation rates in cDNA adjacent to the methylated A. The total number of reads at each site is also shown (Total # reads). The APOBEC1-YTH expressing cells have higher levels of deamination compared to the APOBEC1-YTHmut expressing cells.

TABLE 2 Editing at m⁶A adjacent sites in the ACTB and SPEN mRNAs ACTB mRNA SPEN mRNA % C to T Total # % C to T Total # reads reads reads reads APOBEC1-YTH 29%  207 0% 47 Cell 1 APOBEC1-YTH 0% 177 27%  98 Cell 2 APOBEC1-YTH 19%  367 73%  96 Cell 3 APOBEC1-YTH 68%  380 0% 1 Cell 4 APOBEC1- 0% 288 0% 30 YTHmut Cell 1 APOBEC1- 0% 196 0% 98 YTHmut Cell 2 APOBEC1- 0% 110 0% 0 YTHmut Cell 3 APOBEC1- 0% 244 0% 5 YTHmut Cell 4

Example 3. Transgenic Mice

To generate transgenic mice expressing APOBEC1-YTH and APOBEC1-YTHmut, the APOBEC1-YTH or APOBEC1-YTHmut sequences were first cloned into the pAi9 Rosa26 targeting vector at B stBI and MluI sites. This vector contains a lox-stop-lox cassette preceding the APOBEC1-YTH coding sequence. DNA was purified and used for embryonic stem (ES) cell injections. Following PCR confirmation of transgene-containing ES cell clones, positive clones were injected into morulae to generate chimeric mice. Positive chimeric mice were identified via genotyping and coat color and were mated to wild type mice to confirm germline transmission. Mice found to have germline transgene transmission will then be mated with phiC31 deleter mice (JAX labs) to remove the neomycin resistance cassette, which is flanked by attB/attP sites. The resulting neo-deleted lines will then be expanded and backcrossed for 3-5 generations. These mice will have ubiquitous expression of lox-stop-lox APOBEC1-YTH or YTHmut transgenes. After breeding with Cre driver lines of choice or following viral-mediated delivery of Cre recombinase, the APOBEC1-YTH or APOBEC1-YTHmut fusion protein will be expressed in a tissue of interest.

Sequences

(amino acid sequence of YTHDF2-YTH) SEQ ID NO: 1 PHPVLEKLRSINNYNPKDFDWNLKHGRVFIIKSYSEDDIHRSIKYNIWCSTEHGNKRLDA AYRSMNGKGPVYLLFSVNGSGHFCGVAEMKSAVDYNTCAGVWSQDKWKGRFDVRWI FVKDVPNSQLRHIRLENNENKPVTNSRDTQEVPLEKAKQVLKIIASYKHTTSIFDDFSHY EKRQEEEESVKKERQGRGK (amino acid sequence of YTHDF2-YTH_W432A_W486A) SEQ ID NO: 2 PHPVLEKLRSINNYNPKDFDWNLKHGRVFIIKSYSEDDIHRSIKYNIACSTEHGNKRLDA AYRSMNGKGPVYLLFSVNGSGHFCGVAEMKSAVDYNTCAGVASQDKWKGRFDVRWI FVKDVPNSQLRHIRLENNENKPVTNSRDTQEVPLEKAKQVLKIIASYKHTTSIFDDFSHY EKRQEEEESVKKERQGRGK (amino acid sequence of YTHDF2-YTHmut) SEQ ID NO: 3 GRVFIIKSYSEDDIHRSIKYNIWCSTEHGNKRLDAAYRSMNGKGPVYLLFSVNGSGHFCG VAEMKSAVDYNTCAGVWSQDKWKGRFDVRWIFVKDVPNSQLRHIRLENNENKPVTNS RDTQEVPLEKAKQVLKIIASYKHTTSIFDDFSHYEKRQEEEESVKKERQGRGK (amino acid sequence of YTHDF2-YTHmut2) SEQ ID NO: 4 GRVFIIKSYSEDDIHRSIKYNIACSTEHGNKRLDAAYRSMNGKGPVYLLFSVNGSGHFCG VAEMKSAVDYNTCAGVASQDKWKGRFDVRWIFVKDVPNSQLRHIRLENNENKPVTNS RDTQEVPLEKAKQVLKIIASYKHTTSIFDDFSHYEKRQEEEESVKKERQGRGK (amino acid sequence of YTHDF2-YTH D422N) SEQ ID NO: 5 PHPVLEKLRSINNYNPKDFDWNLKHGRVFIIKSYSENDIHRSIKYNIWCSTEHGNKRLDA AYRSMNGKGPVYLLFSVNGSGHFCGVAEMKSAVDYNTCAGVWSQDKWKGRFDVRWI FVKDVPNSQLRHIRLENNENKPVTNSRDTQEVPLEKAKQVLKIIASYKHTTSIFDDFSHY EKRQEEEESVKKERQGRGK (amino acid sequence of YTHDF1) SEQ ID NO: 6 HPVLEKLKAAHSYNPKEFEWNLKSGRVFIIKSYSEDDIHRSIKYSIWCSTEHGNKRLDSA FRCMSSKGPVYLLFSVNGSGHFCGVAEMKSPVDYGTSAGVWSQDKWKGKFDVQWIFV KDVPNNQLRHIRLENNDNKPVTNSRDTQEVPLEKAKQVLKIISSYKHTTSIFDDFAHYEK RQEEEEVVRKERQSRNKQ (amino acid sequence of YTHDF1 mut) SEQ ID NO: 7 GRVFIIKSYSEDDIHRSIKYSIWCSTEHGNKRLDSAFRCMSSKGPVYLLFSVNGSGHFCG VAEMKSPVDYGTSAGVWSQDKWKGKFDVQWIFVKDVPNNQLRHIRLENNDNKPVTNS RDTQEVPLEKAKQVLKIISSYKHTTSIFDDFAHYEKRQEEEEVVRKERQSRNKQ (amino acid sequence of YTHDF1 D401N) SEQ ID NO: 8 HPVLEKLKAAHSYNPKEFEWNLKSGRVFIIKSYSEDNIHRSIKYSIWCSTEHGNKRLDSA FRCMSSKGPVYLLFSVNGSGHFCGVAEMKSPVDYGTSAGVWSQDKWKGKFDVQWIFV KDVPNNQLRHIRLENNDNKPVTNSRDTQEVPLEKAKQVLKIISSYKHTTSIFDDFAHYEK RQEEEEVVRKERQSRNKQ (amino acid sequence of YTHDF3) SEQ ID NO: 9 VHPVLEKLKAINNYNPKDFDWNLKNGRVFIIKSYSEDDIHRSIKYSIWCSTEHGNKRLDA AYRSLNGKGPLYLLFSVNGSGHFCGVAEMKSVVDYNAYAGVWSQDKWKGKFEVKWI FVKDVPNNQLRHIRLENNDNKPVTNSRDTQEVPLEKAKQVLKIIATFKHTTSIFDDFAHY EKRQEEEEAMRRERNRNKQ (amino acid sequence of YTHDC1) SEQ ID NO: 10 SKLKYVLQDARFFLIKSNNHENVSLAKAKGVWSTLPVNEKKLNLAFRSARSVILIFSVRE SGKFQGFARLSSESHHGGSPIHWVLPAGMSAKMLGGVFKIDWICRRELPFTKSAHLTNP WNEHKPVKIGRDGQEIELECGTQLCLLFPPDESIDLYQVIHKMRHK (amino acid sequence of YTHDC2) SEQ ID NO: 11 PVRYFIMKSSNLRNLEISQQKGIWSTTPSNERKLNRAFWESSIVYLVFSVQGSGHFQGFS RMSSEIGREKSQDWGSAGLGGVFKVEWIRKESLPFQFAHHLLNPWNDNKKVQISRDGQ ELEPLVGEQLLQLWERLPLGEKNTTD (amino acid sequence of rAPOBEC1) SEQ ID NO: 12 MSSETGPVAVDPTLRRRIEPHEFEVFFDPRELRKETCLLYEINWGGRHSIWRHTSQNTNK HVEVNFIEKFTTERYFCPNTRCSITWFLSWSPCGECSRAITEFLSRYPHVTLFIYIARLY HHADPRNRQGLRDLISSGVTIQIMTEQESGYCWRNFVNYSPSNEAHWPRYPHLWVRLY VLELYCIILGLPPCLNILRRKQPQLTFFTIALQSCHYQRLPPHILWATGLK (amino acid sequence of hAICDA) SEQ ID NO: 13 MDSLLMNRRKFLYQFKNVRWAKGRRETYLCYVVKRRDSATSFSLDFGYLRNKNGCHV ELLFLRYISDWDLDPGRCYRVTWFTSWSPCYDCARHVADFLRGNPNLSLRIFTARLYFC EDRKAEPEGLRRLHRAGVQIAIMTFKDYFYCWNTFVENHERTFKAWEGLHENSVRLSR QLRRILLPLYEVDDLRDAFRTLGL (amino acid sequence of hAPOBEC3A) SEQ ID NO: 14 MEASPASGPRHLMDPHIFTSNFNNGIGRUKTYLCYEVERLDNGTSVKMDQHRGFLHNQ AKNLLCGFYGRHAELRFLDLVPSLQLDPAQIYRVTWFISWSPCFSWGCAGEVRAFLQEN THVRLRIFAARIYDYDPLYKEALQMLRDAGAQVSEVITYDEFKHCWDTFVDHQGCPFQP WDGLDEHSQALSGRLRAILQNQGN (amino acid sequence of catalytic domain of ADAR2) SEQ ID NO: 15 QLHLPQVLADAVSRLVLGKFGDLTDNFSSPHARRKVLAGVVMTTGTDVKDAKVISVST GTKCINGEYMSDRGLALNDCHAEIISRRSLLRFLYTQLELYLNNKDDQKRSIFQKSERGG FRLKENVQFHLYISTSPCGDARIFSPHEPILEEPADRHPNRKARGQLRTKIESGQGTIPVRS NASIQTWDGVLQGERLLTMSCSDKIARWNVVGIQGSLLSIFVEPIYFSSIILGSLYHGDHL SRAMYQRISNIEDLPPLYTLNKPLLSGISNAEARQPGKAPNFSVNWTVGDSAIEVINATT GKDELGRASRLCKHALYCRWMRVHGKVPSHLLRSKITKPNVYHESKLAAKEYQAAKA RLFTAFIKAGLGAWVEKPTEQDQFSLT SEQ ID NO: 16 (SGSETPGTSESATPE) SEQ ID NO: 17 (SGSETPGTSESATPES) SEQ ID NO: 18 ((GGGGS)₃) SEQ ID NO: 19 ((GGGGS)₁₀) SEQ ID NO: 20 ((GGGGS)₂₀) SEQ ID NO: 21 (A(EAAAK)₃A) SEQ ID NO: 22 (A(EAAAK)₁₀A) SEQ ID NO: 23 (A(EAAAK)₂₀A) SEQ ID NO: 24 (PKKKRKV) SEQ ID NO: 25 (LPPLERLTL) SEQ ID NO: 26 (MDPVVVLGLCLSCLLLLSLWKQSYGGG) SEQ ID NO: 27 (EQKLISEEDL) SEQ ID NO: 28 (GKPIPNPLLGLDST) SEQ ID NO: 29 (IPNPLLGLD) SEQ ID NO: 30 (DYKDDDDK) SEQ ID NO: 31 (DYKDHDGDYKDHDIDYKDDDDK) SEQ ID NO: 32 (ISLIAALAVDHVIGMETVMPWNLPADLAWFKRNTLNKPVI MGRHTWESIGRPLPGRKNIILSSQPSTDDRVTWVKSVDEAIAACGDVPEIMVIGGGRVYE QFLPKAQKLYLTHIDAEVEGDTHFPDYEPDDWESVFSEFHDADAQNSHSYCFEILERR) SEQ ID NO: 33 (YPYDVPDYA) SEQ ID NO: 34 (GGAGTTGATTGAGGTAAAGCG) SEQ ID NO: 35 (GUUCUUCUGUGGACUGUG) SEQ ID NO: 36 (AGACTCCCGGGACCTCAGAG) SEQ ID NO: 37 (ACTCCCGGGACCTCAGAGTCCGCCACACCAGAAGGCCGGGTTTTCATCATTAAG) SEQ ID NO: 38 (CGGGTTTAAACTCAGGCGTAGTC) SEQ ID NO: 39 (GTGGGGGCGATCTTTATTGTGGCGG) SEQ ID NO: 40 (TGTGCAATCAAAGTCCTCGGCCAC) SEQ ID NO: 41 (GCCAATGCTGTCTGGTTGCGCC) SEQ ID NO: 42 (GGAGGCTTCTGCGGTTCTGGAG) SEQ ID NO: 43 (CAGCAAGCAGGAGTATGACGAGTC) SEQ ID NO: 44 (CATGCCAATCTCATCTTG) SEQ ID NO: 45 (CATGTACGTTGCTATCCAGGC) SEQ ID NO: 46 (CTCCTTAATGTCACGCACGAT) SEQ ID NO: 47 (CGAAGATCCCCACGTGTAAAGACTAC) SEQ ID NO: 48 (CATCCTGCTCACCTCTTTGAGG) SEQ ID NO: 49 (GTGTTAGATGATGTCTCCATTCGGAG) SEQ ID NO: 50 (CACTTTCCTCCTGTATGAGCGG) SEQ ID NO: 51 (GCCAAGAGGCAACACACCAAC) SEQ ID NO: 52 (CGGTTTCTCTCGGTCTGTTTTCC) SEQ ID NO: 53 (CAGAAGGCGACAACACAGCAACACC) SEQ ID NO: 54 (TACTTCCAATCCAATGCA) SEQ ID NO: 55 (TTATCCACTTCCAATGTTATTA) SEQ ID NO: 56 (TACTAGGACGCACCTTA) SEQ ID NO: 57 (TACTAGGATGCACCTTA) E. coli codon optimized APOBEC1-YTH for protein purification: SEQ ID NO: 58 ATGAGCAGCGAAACCGGTCCGGTGGCGGTTGACCCGACCCTGCGTCGTCGTATTGA GCCGCACGAGTTCGAAGTGTTCTTTGATCCGCGTGAGCTGCGTAAGGAAACCTGCCT GCTGTACGAAATTAACTGGGGTGGCCGTCACAGCATCTGGCGTCACACCAGCCAGA ACACCAACAAGCACGTTGAGGTGAACTTCATCGAAAAATTTACCACCGAGCGTTACT TCTGCCCGAACACCCGTTGCAGCATTACCTGGTTTCTGAGCTGGAGCCCGTGCGGTG AATGCAGCCGTGCGATCACCGAGTTCCTGAGCCGTTATCCGCACGTTACCCTGTTTA TCTACATTGCGCGTCTGTATCACCACGCGGACCCGCGTAACCGTCAAGGTCTGCGTG ATCTGATCAGCAGCGGCGTGACCATCCAGATTATGACCGAGCAAGAAAGCGGTTAC TGCTGGCGTAACTTCGTTAACTATAGCCCGAGCAACGAAGCGCATTGGCCGCGTTAC CCGCACCTGTGGGTGCGTCTGTACGTTCTGGAGCTGTATTGCATCATTCTGGGCCTGC CGCCGTGCCTGAACATTCTGCGTCGTAAGCAGCCGCAACTGACCTTCTTTACCATCG CGCTGCAGAGCTGCCACTACCAACGTCTGCCGCCGCACATTCTGTGGGCGACCGGTC TGAAGAGCGGCAGCGAAACCCCGGGTACCAGCGAAAGCGCGACCCCGGAGCCGCA CCCGGTGCTGGAGAAACTGCGTAGCATCAACAACTATAACCCGAAGGACTTCGATT GGAACCTGAAACACGGTCGTGTTTTTATCATTAAGAGCTACAGCGAAGACGATATCC ACCGTAGCATTAAATATAACATCTGGTGCAGCACCGAGCACGGCAACAAGCGTCTG GACGCGGCGTACCGTAGCATGAACGGTAAAGGCCCGGTGTATCTGCTGTTCAGCGTT AACGGTAGCGGCCACTTTTGCGGTGTGGCGGAAATGAAAAGCGCGGTTGATTACAA CACCTGCGCGGGTGTGTGGAGCCAGGACAAGTGGAAAGGCCGTTTCGATGTTCGTT GGATTTTTGTGAAGGACGTTCCGAACAGCCAACTGCGTCACATCCGTCTGGAGAACA ACGAAAACAAACCGGTGACCAACAGCCGTGATACCCAGGAAGTGCCGCTGGAAAA GGCGAAACAAGTTCTGAAGATCATTGCGAGCTACAAACACACCACCAGCATCTTCG ACGATTTTAGCCACTATGAGAAGCGTCAGGAAGAGGAAGAGAGCGTGAAGAAGGA GCGTCAAGGTCGTGGCAAACTGGAGTACCCGTATGACGTTCCGGATTATGCGTAAAT TGGAAGTGGATAA E. coli codon optimized APOBEC1-YTHmut for protein  purification: SEQ ID NO: 59 ATGAGCAGCGAAACCGGTCCGGTGGCGGTTGACCCGACCCTGCGTCGTCGTATTGA GCCGCACGAGTTCGAAGTGTTCTTTGATCCGCGTGAGCTGCGTAAGGAAACCTGCCT GCTGTACGAAATTAACTGGGGTGGCCGTCACAGCATCTGGCGTCACACCAGCCAGA ACACCAACAAGCACGTTGAGGTGAACTTCATCGAAAAATTTACCACCGAGCGTTACT TCTGCCCGAACACCCGTTGCAGCATTACCTGGTTTCTGAGCTGGAGCCCGTGCGGTG AATGCAGCCGTGCGATCACCGAGTTCCTGAGCCGTTATCCGCACGTTACCCTGTTTA TCTACATTGCGCGTCTGTATCACCACGCGGACCCGCGTAACCGTCAAGGTCTGCGTG ATCTGATCAGCAGCGGCGTGACCATCCAGATTATGACCGAGCAAGAAAGCGGTTAC TGCTGGCGTAACTTCGTTAACTATAGCCCGAGCAACGAAGCGCATTGGCCGCGTTAC CCGCACCTGTGGGTGCGTCTGTACGTTCTGGAGCTGTATTGCATCATTCTGGGCCTGC CGCCGTGCCTGAACATTCTGCGTCGTAAGCAGCCGCAACTGACCTTCTTTACCATCG CGCTGCAGAGCTGCCACTACCAACGTCTGCCGCCGCACATTCTGTGGGCGACCGGTC TGAAGAGCGGCAGCGAAACCCCGGGTACCAGCGAAAGCGCGACCCCGGAGGGTCG TGTTTTTATCATTAAGAGCTACAGCGAAGACGATATCCACCGTAGCATTAAATATAA CATCTGGTGCAGCACCGAGCACGGCAACAAGCGTCTGGACGCGGCGTACCGTAGCA TGAACGGTAAAGGCCCGGTGTATCTGCTGTTCAGCGTTAACGGTAGCGGCCACTTTT GCGGTGTGGCGGAAATGAAAAGCGCGGTTGATTACAACACCTGCGCGGGTGTGTGG AGCCAGGACAAGTGGAAAGGCCGTTTCGATGTTCGTTGGATTTTTGTGAAGGACGTT CCGAACAGCCAACTGCGTCACATCCGTCTGGAGAACAACGAAAACAAACCGGTGAC CAACAGCCGTGATACCCAGGAAGTGCCGCTGGAAAAGGCGAAACAAGTTCTGAAGA TCATTGCGAGCTACAAACACACCACCAGCATCTTCGACGATTTTAGCCACTATGAGA AGCGTCAGGAAGAGGAAGAGAGCGTGAAGAAGGAGCGTCAAGGTCGTGGCAAACT GGAGTACCCGTATGACGTTCCGGATTATGCGTAAATTGGAAGTGGATAA SEQ ID NO: 60 (METDTLLLWVLLLWVPGSTGD) deaminase domain of rAPOBEC1 SEQ ID NO: 61 RRRIEPHEFEVFFDPRELRKETCLLYEINWGGRHSIWRHTSQNTNKHVEVNFIEKFTTER YFCPNTRCSITWFLSWSPCGECSRAITEFLSRYPHVTLFIYIARLYHHADPRNRQGLRDLI SSGVTIQIMTEQESGYCWRNFVNYSPSNEAHWPRYPHLWVRLYVLELYCIILGLPPCLNI LRRKQPQLTFFTIALQSCHYQRLPPHILWATGLK deaminase domain of hAICDA SEQ ID NO: 62 LMNRRKFLYQFKNVRWAKGRRETYLCYVVKRRDSATSFSLDFGYLRNKNGCHVELLF LRYISDWDLDPGRCYRVTWFTSWSPCYDCARHVADFLRGNPNLSLRIFTARLYFCEDRK AEPEGLRRLHRAGVQIAIMTFKDYFYCWNTFVENHERTFKAWEGLHENSVRLSRQLRRI LLPL deaminase domain of hAPOBEC3A SEQ ID NO: 63 TSNFNNGIGRHKTYLCYEVERLDNGTSVKMDQHRGFLHNQAKNLLCGFYGRHAELRFL DLVPSLQLDPAQIYRVTWFISWSPCFSWGCAGEVRAFLQENTHVRLRIFAARIYDYDPLY KEALQMLRDAGAQVSIMTYDEFKHCWDTFVDHQGCPFQPWDGLDEHSQALSGRLR catalytic domain of ADAR2 SEQ ID NO: 64 MDSLLMNRREFLYQFKNVRWAKGRRETYLCYVVKRRDSATSFSLDFGYLRNKNGCHV ELLFLRYISDWDLDPGRCYRVTWFISWSPCYDCARHVADFLRGNPNLSLRIFTARLYFCE AGRREPEGLRRLHRAGVQIAIMTFKDYFYCWNTFVENHGRTFKAWEGLHENSVRLSRQ LRRILL 

What is claimed is:
 1. A fusion protein comprising an N⁶-methyladenosine (m⁶A) binding domain of a YT521-B homology (YTH) domain-containing protein fused to a catalytic domain of a cytidine deaminase or a catalytic domain of an adenosine deaminase.
 2. The fusion protein of claim 1, wherein the m⁶A binding domain is fused to the catalytic domain via a peptide linker.
 3. The fusion protein of claim 1, wherein the m⁶A binding domain comprises a polypeptide having at least 95% identity to SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 10, or SEQ ID NO:
 11. 4. The fusion protein of claim 1, wherein the catalytic domain comprises a polypeptide having at least 95% identity to SEQ ID NO 12 or a catalytic fragment thereof, SEQ ID NO: 13 or a catalytic fragment thereof; SEQ ID NO: 14 or a catalytic fragment thereof; or SEQ ID NO:
 15. 5. The fusion protein of claim 1, wherein the fusion protein further comprises a localization element.
 6. The fusion protein of claim 5, wherein the localization element is fused to the N-terminus or the C-terminus of the fusion protein.
 7. A recombinant nucleic acid encoding the fusion protein of claim
 1. 8. A DNA construct comprising a promoter operably linked to the recombinant nucleic acid of claim
 7. 9. The DNA construct of claim 8, wherein the promoter is an inducible promoter.
 10. A vector comprising the recombinant nucleic acid claim
 7. 11. A host cell comprising the recombinant nucleic acid of claim
 7. 12. A host cell comprising the vector of claim
 9. 13. The host cell of claim 11, wherein the cell is a mammalian cell.
 14. A non-human transgenic animal comprising the host cell of claim
 13. 15. A method for detecting m⁶A residues in the RNA produced by one or more cells comprising: a) isolating RNA from one or more cells that expresses the fusion protein of claim 1; b) amplifying one or more target sequences in the isolated RNA; and c) sequencing the one or more target sequences to identify cytidine to uridine deamination at sites adjacent to m⁶A residues, thus detecting the m⁶A residues in the RNA of the one of more cells.
 16. The method of claim 15, wherein a recombinant nucleic acid encoding the fusion protein is introduced into the one or more cells prior to isolating RNA from the cell.
 17. The method of claim 15, wherein the fusion protein is stably or transiently expressed in the cell.
 18. The method of claim 15, wherein the one or more target sequences are amplified by reverse transcriptase polymerase chain reaction (RT-PCR).
 19. The method of claim 15, wherein the RNA comprises one or more RNAs selected from the group consisting of messenger RNA (mRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), a regulatory RNA, a transfer-messenger RNA (tmRNA), ribosomal RNA (rRNA), microRNA (miRNA), long noncoding RNA (lncRNA) or circular RNA (circRNA).
 20. A method for detecting m⁶A residues in the RNA produced by one or more cells comprising: a) fixing a cell or tissue that expresses the fusion protein of claim 1; and b) detecting cytidine to uridine deamination in the RNA, wherein cytidine to uridine deamination is detected via mutation-sensitive in situ hybridization.
 21. A method for detecting m⁶A residues in a biological sample comprising: a) isolating RNA from a biological sample; b) contacting the RNA with the fusion protein of claim 1; c) amplifying one or more target sequences in the RNA; and d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues in the one or more target sequences, thus detecting the m⁶A residues in the one or more target sequences.
 22. The method of claim 21, wherein the RNA comprises one or more RNAs selected from the group consisting of messenger RNA (mRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), a regulatory RNA, a transfer-messenger RNA (tmRNA), ribosomal RNA (rRNA), microRNA (miRNA), long noncoding RNA (lncRNA) or circular RNA (circRNA).
 23. The method of claim 21, wherein the biological sample is a biopsy.
 24. The method of claim 21, wherein the one or more target sequences are amplified by reverse transcriptase polymerase chain reaction (RT-PCR).
 25. The method of claim 21, wherein cytidine to uridine deamination is identified by sequencing the one or more target sequences.
 26. A method for diagnosing a disease in a subject comprising: a) isolating RNA from a biological sample; b) contacting the RNA with the fusion protein of claim 1; c) amplifying one or more target sequences in the RNA; and d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues in the one or more target sequences to identify a pattern of m⁶A residues, a difference in the pattern of m⁶A residues as compared to a reference pattern of m⁶A residues from a reference population(s), indicating the subject has or is at risk for developing the disease.
 27. The method of claim 26, wherein the disease is selected from the group consisting of cancer, an autoimmune disorder, a neurodegenerative disorder, and a viral infection.
 28. The method of claim 26, wherein the biological sample is a biopsy.
 29. The method of claim 26, wherein the one or more target sequences are amplified by reverse transcriptase polymerase chain reaction (RT-PCR).
 30. The method of claim 26, wherein cytidine to uridine deamination is identified by sequencing the one or more target sequences.
 31. A method for determining the efficacy of a selected treatment for a disease in a subject comprising: a) isolating RNA from a biological sample from the subject before the selected treatment; b) contacting the RNA with the fusion protein of claim 1, c) amplifying one or more target sequences in the RNA; d) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues to identify a pattern of m⁶A residues in the one or more target sequences; e) treating the subject with the selected treatment, f) isolating RNA from a biological sample from the subject at one or more times after administration of the selected treatment; g) amplifying one or more target sequences in the RNA of step f); h) identifying cytidine to uridine deamination at sites adjacent to m⁶A residues to identify a pattern of m⁶A residues in the one or more target sequences of step g); and i) comparing the pattern of m⁶A residues identified in step (d) and (h) to determine whether the pattern of m⁶A residues is the same or whether the m⁶A residues detected in step (d) or (h) is more similar to a reference pattern, a pattern of m⁶A residues in step (h) more similar to the reference pattern indicating the selected treatment is effective for treating the disease in the subject.
 32. The method of claim 31, wherein the disease is selected from the group consisting of cancer, an autoimmune disorder, a neurodegenerative disorder, and a viral infection.
 33. The method of claim 31, wherein the biological sample is a biopsy.
 34. The method of claim 31, wherein the one or more target sequences are amplified by reverse transcriptase polymerase chain reaction (RT-PCR).
 35. The method of claim 31, wherein cytidine to uridine deamination is identified by sequencing the one or more target sequences.
 36. A kit comprising: a) the fusion protein of claim 1; and b) one or more primers for amplification of one or more target RNA sequences. 